Birdwatching: Take two conservationists, a couple of thousand birds, and one windswept rock, with only a lighthouse for shelter. It could be the scene of a survival test - both for the endangered species of tern and for the duo charged with their protection. Sheila Ryan reports from Rockabill.

It sounds like a particularly grim reality TV experiment: put two birdwatchers on two acres of uninhabited rocks for the summer, completely at the mercy of the weather and only each other for company, and monitor the results.

As our boat pulls up alongside the tiny pier of Rockabill, a pair of tiny islands off the north coast of Dublin, the jokes begin. Will we tell them the war in Afghanistan is over?

It is BirdWatch Ireland which employs two wardens to live and work on Rockabill for three months in the summer. The chosen two this year are conservationists David Patrick, from England and Mary Robinson from Co Donegal.

"When the ocean's really raging it's amazing out here," says David. "We've had a few fairly wild weeks where you really get to appreciate how strong the sea can be, when there are waves crashing over everything and spray hitting your bedroom window." Unlike reality TV contestants, Mary and David have a real reason to be here.

More than 1,300 pairs of terns nest on Rockabill every summer, and it's the job of the wardens to observe and protect them.

"We monitor the total number of nests on the island of each species, and in certain study areas we look and see how many eggs are laid in each nest," Mary says. "We follow those nests to see how many of those eggs actually hatch and we then try and ring all of those chicks and see how many of them survive and see what kind of success rate they have."

Among the residents are more than 600 pairs of Roseate Terns, making Rockabill the largest colony in Europe. Part of the wardens' job is to help increase the numbers of these endangered birds by providing nest boxes and protection from predators.

Terns nest on the ground, and clusters of eggs are strewn about the steps and paths of the main island, making it very easy to disturb them. There are no rats on Rockabill and public access is restricted during the breeding season, making it an ideal nesting ground for the birds. But Rockabill is not quite as hospitable for its human occupants.

David and Mary live in an old lighthouse keeper's house. It's a listed building and the view is second to none. But as the lighthouse has been automated since the 1980s, the electricity generator doesn't kick in until after dark. Although they can see Skerries from the island, this summer's weather has meant problems getting supplies.

"The weather has been so bad that getting a boat up to the island has been difficult," says Mary. "Getting on and off has been a real challenge," adds David.

But it's not all doom and gloom. When the weather is good, the fisherman who brings their groceries might also share his catch with them, and then they have a feast of crabs and fresh fish.

Ironically, bad weather can be a bonus for the Rockabill wardens. If the weather is cloudy, the electricity comes on earlier, and if it's foggy, the generator automatically starts in order to power the fog horn. It was fog that allowed them to see Ireland's exit from the World Cup.

"Just as the game was about to start we got really bad weather, and the fog horn came on, so we got electricity for that," David says.

The isolation has made it difficult to keep in touch with friends. "If you want to write letters you've got to wait until the fisherman can come in," Mary says. "Especially this year, with the weather being so bad, sometimes you have a letter waiting for 10 days before you can actually give it to him to send off." She adds that what she misses most is e-mail, and being able to go for a walk and bumping into people.

David also misses walking. He has a passion for hiking and climbing and describes himself as hyperactive. The blue outline of the Mourne Mountains is tantalisingly visible on the northern horizon, but this summer he can't go further than a couple of hundred metres from his front door. Mary jokes that she got worried when she saw her new colleague and house mate unloading a punch bag from the boat when they moved over in May. But, along with a skipping rope, this helps David to burn off his excess energy.

While David is delivering blows to his punch bag, Mary is more likely to be painting or doing yoga, suggesting potential for a clash of personalities. However, both insist that getting along has not been a problem, even in such close quarters.

"We both enjoy our own company," David says, so they tend not to be sitting arguing over the TV channels in the evening. Mary adds: "We're both pretty mellow." David points out that they spend most days working separately. "We don't really see each other all that much," he laughs.

They see a lot of the birds though. Approaching Rockabill, the lighthouse looks like a snow globe, with thousands of flecks of white swirling around it. The wardens' work clothes are splattered with evidence that the birds have a good aim. For an endangered species the terns are alarmingly feisty, and I was warned to put plenty of padding under my hat in case of attack.

For Mary and David, being this close to nature is what makes it all worthwhile. It's hard to believe that Rockabill is only six kilometres from shore. Seals pop their heads up next to the tiny pier and the humped backs of porpoises make a brief appearance further out to sea.

By the end of August, the fluffy chicks that are now staggering from their eggs will be ready to stretch their wings. When all the chicks have fledged it will be time for the wardens to leave their island home too. They will be sad to see the birds go, but admit the prospect of a longer stay on Rockabill would be daunting.

"Living on an island like this you can't escape yourself, so it's an interesting psychological experience living out here. Staying here over winter would be interesting," David says.

Mary is more emphatic. "This summer the weather has been much worse than previous years so we've really gotten a taste of what the weather would be like in the winter. I can imagine that if it was like that for a couple of months it would be pretty dismal," she concludes.