In spite of its dominant position at the mouth of Ireland's major waterway, Limerick is a destination that boat-owners have traditionally avoided. But following the opening of a city-centre navigation system yesterday, it has made the Killaloe to Limerick passage a novel city cruise.

Seasoned yachtsmen, at home in the wave troughs of Atlantic swells, balk at the prospect of navigating the 22 km stretch from Limerick to Killaloe, the southern access point of the river Shannon.

Strong currents, shallows, rapids, low bridges and fast-ebbing tides were so daunting that the number of pleasure craft going through Ardnacrusha's twolock system was only 100 boats in the 1998/99 season.

Traffic at the southern end of Europe's leading pleasure waterway had all but dried up. It was a river stretch which had many obstacles, but was otherwise navigable from Ballybunion to Ballyshannon.

Experienced visiting boat-owners had to make precise tidal predictions and avail of specialist pilots to take them through the Limerick passage of the Abbey river because water-level differences between Killaloe and Limerick ranged by over a 100 feet.

Power station discharges at Ardnacrusha created dangerous flows and water and air draught levels on Limerick city bridges were critically low. It made navigation possible for just minutes in any given day. For pleasure boaters the experience was more a job of work than of leisure.

The decision to regenerate this urban waterway came about two years ago by visionary engineers who carried out an interceptor sewer project. It involved laying 1,100 metres of pipe on the river bed. The project combined the effort of three public agencies: Limerick Corporation, Waterways Ireland and Shannon Development.

The by-product of the massive sewerage work is an inspirational feat for waterway enthusiasts. Engineers built a weir on top of the new drainage pipe as it emerged from the Abbey river mouth, ensuring a minimum depth of 1.7 metres could be maintained for boating traffic that can now pass through the city centre.

Then they dredged the Abbey river, restoring ancient locks and providing city-centre mooring facilities. The result is set to add further buoyancy to a boat hire business, a flagship of the waterway tourism product in Ireland since the 1960s.

The trip from Killaloe to Limerick involves such changes in scenery over the 22 km downstream that it is a wonder so many aspects of Irish waterscapes can be rolled into one short passage.

Such is the water rushing from Lough Derg under Killaloe bridge that ESB engineers reckon the flow can reach 600 tonnes per second during a winter flood.

This bridge area opens up into the lower lake where small craft navigate by following a series of marks.

Going downstream, vessels are required to keep black marks on their port (left) side and red on their starboard side. The reverse applies on an upstream journey. After a 5 km lake trip, the first evidence of man's ability to tame the Shannon is the Parteen Villa weir.

Access through the weir is via a guillotine gate to a 11.6 km stretch, known as the headrace that precedes the two locks of Ardnacrusha power station.

It is advisable to book ahead to lock through this massive structure, not because the process can take an hour to allow boats descend 100 feet into the tailrace, but because if the ESB are running two or three turbines it can double the length of time it will take to make the journey.

In the face of this, boats need a minimum speed of five or six knots to accomplish the trip.

No longer seen as an obstacle to the journey, the locking process is an educational feature of the trip. It is an opportunity to see the hydroelectric system that produced 87 per cent of Ireland's electricity in 1936 at first hand. Today it provides about 3 per cent.

Once the lock-keeper opens the second gate you enter the 2.4 km tailrace and the Shannon is yours to explore.

There are pleasant anchorages and - amazingly on the outskirts of a major city - an abundance of wildlife, including heron nests.

IT IS important to follow the buoyage system and the journey downstream can be tracked by noting a variety of bridges along the route. Many carry airdraft gauges to give boats with high superstructures a guide of what height is available.

Soon after passing under the Limerick to Ennis railway bridge, you turn to port and enter the Abbey river, a 1 km stretch where substantial dredging has made passaging much safer.

Baal's Bridge is the lowest of all bridges encountered and in spite of improvements headroom is at a premium.

Limerick sailor Edgar Heenan, who published navigational notes on the passage, remarks: "The current here can be quite strong during spring tides, when there has been a lot of recent heavy rain, or when two or three turbines are in operation in Ardnacrusha. In the unlikely event of all three occurring at the same time, exercise extra caution."

From this point you cruise the city centre and developers have in place 70 mooring pontoons at city locations.

You can berth overnight or continue past the new weir and head for the Atlantic Ocean through refurbished Sarsfield Lock and the estuary.