A museum in Denmark has reconstructed an Irish-built Viking longship which is on its way to Dublin, writes Rosita Boland

The sea can keep secrets for a long time beneath its extensive and ever-changing surface. For centuries, fishermen in Denmark's Roskilde Fjord found pieces of wood in their nets when they hauled them up in the vicinity of Skuldelev. The fishermen focused on fishing, their visceral, primary task, but kept the lore of their inanimate finds alive. They charted their own aural map of what lay where beneath the surface, and passed it on down the generations.

"The fishermen always knew something was there," says Ole Crumlin-Pedersen, former director of the Centre for Maritime Archaeology at Roskilde, and the author of many books on the subject. In 1957, Crumlin-Pedersen was a member of a team that dived near Skuldelev in Roskilde fjord to the location where the fishermen had long been bringing wood to the surface in their nets.

"We thought what we'd find would be the remains of a medieval ship," explains Crumlin-Pedersen, known in Denmark as the grand old man of maritime archaeology. What they found instead was a wholly unexpected archaeological time-capsule: the wrecks of five Viking ships, all close together, lying at the bottom of an old shipping channel through the fjord. The ships had been deliberately scuttled in the 11th century in order to block the channel against invaders. Over the centuries, parts of them had slowly rotted, and come adrift; these were the pieces of wood that ended up in the fishermen's nets.

It took five years for the shipwrecks to be removed from the water. Preservation, and the huge task of jigsawing together the incomplete pieces of five differently built ships, took another 25 years. Today, they are all on show in the specially built Viking Ship Museum in Roskilde, where there are several associated displays: scale models of the ships, a boatyard, a wood-preservation workshop, and information on Danish maritime archaeology. This museum is regarded as being the best international source of information on Viking ships.

The museum building itself is listed: a clever concrete box of 1960s brutalist architecture that brilliantly showcases its contents. The central part of the building is floored with water-washed stones, on which the stark remains of the five ships are displayed on stands, looking rather like the fantastical skeletons of whales. Some ships are more intact than others. One is 75 percent complete, one is half complete and one, Skuldelev 2, an ocean-going longship, only has 25 percent of its original timbers remaining.

The ships are framed against a view of Roskilde Fjord, which laps right up to the window. If you stand behind Skuldelev 2, you can see through the missing parts of its structure into the fjord beyond. And on this particular May day, you could stand in the museum, look through the fragments of this 11th-century Viking ship, and see sailing in the fjord the Skuldelev 2's 21st-century twin shadow: an extraordinary reconstruction 10 years in the making.

Modern technology has enabled scientists to make discoveries about the age and origin of the timbers that the boats were constructed from. While four of the five ships were made locally of wood from Norway and Denmark, the fifth and biggest, the warrior longship Skuldelev 2, is made of Irish oak from Wicklow. Carbon dating and studying the rings of the wood have revealed that the boat was built in Ireland in 1042. Such was its size - 30 metres - that it could only have been built by a powerful chieftain, who used it for purposes of war. A couple of decades after it was built in Ireland, it ended up at the bottom of Roskilde Fjord, where it lay undiscovered with the others for close to a millennium.

The Viking Ship Museum had already built scale models of the ships it found in the fjord. But several years ago, the museum came up with a far more ambitious plan: to build a full-size reconstruction of the longship. Not only that, but the ambition was to crew it with the 65 people needed and then sail it back to Ireland, where the original ship had come from. On July 1 the reconstructed ship, named the Sea Stallion from Glendalough, will leave Denmark. It's due to arrive in Dublin on August 14. After two days on the docks, it will be moved to Collins Barracks, where a crane will lift it over the buildings into the courtyard. It will remain on display until next summer, when the crew will make a return journey to Denmark.

On this hot May day, the Sea Stallion is combining a training run with showing off the boat under sail in the fjord to several members of the international press.

When it comes to open sea-going boats, we have a different sense of scale in the 21st century. What was in the 11th century a fast, intimidating, sophisticated longship warrior boat, owned by a powerful chieftain, looks very different to modern eyes. On the quayside, the Sea Stallion looks like a very large and very vulnerable row boat. There are no cabins, no shelter, no engines. This is where 65 people will row, sail, eat, and sleep - and, undoubtedly, argue, laugh and stress out - for the seven-week voyage on open seas from Denmark, via the Shetlands, down the west coast of Scotland, and on to Dublin. Although they will camp when they can make land, the crew will often be sailing for up to six days at a time on the open seas.

There's no time for niceties or formal welcomes when it comes to boarding. It's a matter of piling on, stepping over benches, and finding somewhere to stand where you can be in the way as little as possible. With a huge sail, the yardarm defines most of the length of the boat. Visitors to the museum stand watching on the quayside as the crew of 65 take their places at the oars in this piece of reconstructed history, and the ship moves out into the fjord, the timbers creaking loudly.

The man at the ship's prow giving commands in Danish - although some crew-members are foreigners, all commands at sea are given in Danish - is Carsten Hviid, the captain.

"No one has sailed a ship like this in 900 years," he says cheerfully, explaining what initially drew him to to the project. All the crew are volunteers, and there are 120 in total, of which half are reserve for the main crew. They have been in training for years. The oldest is 64 and the youngest is 16, the son of another crew-member. "It's a fantastic chance to learn about sailing a square-rigged boat."

Since there was only a quarter of the original ship left, reconstructing it has been, to a certain degree, guesswork. "It would have taken less than a year to build the original ship, because the knowledge of how to do it was passed down among the boatbuilders," Hviid says. "It took us four years." The boat was built using copies of traditional tools, and was constructed entirely by hand in the museum's boatyard, every part planed and carved and hewn. The huge sail that gives the ship its speed would originally have been at least as valuable as the ship itself, and taken even longer to make.

We're out on the open water now. Commands are given to stow the oars, and the sail is hauled by 12 people. Even with the sail up, the impression on board is how tight, almost claustrophobic, the space is.

Every one of the 65 crew members has a designated space on board, where they sit and where they'll sleep. They take turns to sleep in between keeping watch. This is one of their last sail training weekends before the voyage to Ireland. There was a month-long training voyage to Norway last summer, but the weather was obstinately fine and there is some real worry about how the boat will perform in the unpredictable waters of the North Atlantic, should a storm blow up en route.

While most of the crew are Danish, there are also members from 10 other countries, including the US, Canada, Australia, the Netherlands, Germany and Norway. There is just one Irish crew member, Tríona Micholl (24), from Leixlip, Co Kildare. She's been in training for the last two years, coming over from Ireland at her own expense for sail-training weekends and the summer voyages. She's doing a PhD in archaeology in UCD on early medieval settlements in Ireland, which is a large part of the reason for her interest in the ship.

"I really wanted the challenge of this," she explains. "Not just the physical challenge of sailing a Viking vessel, but the social challenges; of getting on with so many people in such a small space. We all do have worries about something going wrong with the boat. When things go wrong on an open-deck vessel, they go wrong very quickly, but we don't dwell on it, otherwise none of us would do it. Dublin is the Holy Grail: arriving in Dublin has assumed mythical proportions for the rest of the crew at this stage."

THE PROJECT HAS BEEN funded in a number of ways, mainly through private donation and foundations in Denmark, including €1.3 million from the Tuborg Foundation. Here, the Department of Arts, Sports and Tourism is allocating €200,000 from its special-projects fund. There are many strands to the voyage to Ireland and back. Apart from the experiment of sailing the boat, testing the sea-worthiness of the reconstruction, and logging the work of the crew, there is also the fact that this is a major collective adventure, and as much a social experiment as a historical one.

The project has bought a certain amount of satellite time, and the chef, nurse and a crew member will be blogging daily. They're hoping a webcam will operate for a significant portion of time, and the ship's position will be updated daily on an electronic map, with links to meteorological information. Such are the media-driven times we live in that Danish publishers have already lined up books by crew members on the experience, which will be published in the autumn.

The crew includes photographers and journalists, and the support boat will carry camera crews, some filing news updates for television, and others filming for a documentary. Nothing will go unobserved or unrecorded.

The Sea Stallion will be followed at all times by a support boat, which, if necessary, is capable of rescuing and taking aboard all crew members. It's not only the weather which is an unknown quantity. In living memory, no other boat of this kind has been tested in open seas. The boatbuilders, who had only a quarter of the original ship to go on for a template, do not know how seaworthy it will prove when tested. Each day, they will be examining the ship for signs of strain, and making urgent repairs as needed.

Jens Knudsen, at 64, is the oldest crew member and also the chef. "We will have enough food on board at any one time for 10 days, and water for eight days." There won't be any alcohol on board, for obvious reasons, although O'Donoghue's pub on Merrion Row has already been booked for the night they arrive in Dublin. Nor will they be cooking with an open fire, as their Viking counterparts would have done. "We are in a Viking ship, but using modern equipment - lifejackets, rainproofs, satellite, gas-cookers. We had to draw the line somewhere in reconstructing the experience." There is one toilet, located in the middle of the shop, surrounded by a sail. "It has a mahogany seat!" Knudsen laughs.

July 1, the day on which the Sea Stallion from Glendalough will set sail to Ireland, was chosen for a specific reason. It will be the 50th anniversary of the date on which Ole Crumlin-Pedersen first discovered the submerged timbers of the original ship, and he will be the guest of honour on the quay at Roskilde to see off its remarkable 21st-century counterpart.

June 28: A free exhibition, Sea Stallion, Dublin's Viking Warship Comes Home, will open at the National Museum of Ireland, Collins Barracks, Dublin. Track the voyage from Roskilde to Dublin via the web and learn about the discovery of the archaeological remains of the construction of the replica warship. Admission is free

Sea Stallion a celebration

July 1: Sea Stallion departs from Roskilde, Denmark

August 14-15: Welcoming ceremony for Sea Stallion and a two-day celebration at Custom House Quay

August 17: Sea Stallion arrives in Collins Barracks and is lifted into the centre of Clarke Square where it can be viewed by visitors.

There will be various Viking-themed activities and workshops at the museum that weekend. Both the Sea Stallion and the accompanying exhibition will remain on display at Collins Barracks until next summer, when the ship will be sailed back to Roskilde.

For further information see www.museum.ie, www.seastallion.dk and www.vikingeskibsmuseet.dk