A golden eagle is hovering over the long grasses of a hillside, talons ready to strike at an indistinct form below. Suddenly the harassed prey unfolds, leaping up and snapping at the bird: a bright red fox in a classic - and effective - act of defiance. In this dramatic image, captured earlier this year by cameraman Brian Black, a whole chapter is restored to Irish natural history.

The fierce clash between arch-predators celebrates the success of the reintroduction programme that now has 34 eagles in the skies of Donegal, some also roaming out to prospect the mountains of adjoining counties. Eagles thought to be from Glenveagh have now been seen down the west coast as far away as Achill and Kerry.

Of all the island's bird extinctions, none was as deliberate as the eradication of the eagle in the latter half of the 19th Century. In the mid 1800s, one could have seen a dozen in a day in the mountains of Kerry, or watched them swooping after hares or grouse from Connemara to Donegal. After 50 years of persecution, with sheep ranchers and gamekeepers using rifles, traps and strychnine, and collectors vying for the eggs, the last native eagles nested above Glenveagh in 1910 and on the north Mayo cliffs in 1912. Ireland is the only country in the world to have lost its eagles in such recent times.

Their restoration was the patient dream of many naturalists, especially when, in an age of conservation, the eagles surviving in Scotland were helped to recover their numbers. With almost 1,000 birds today, Scotland has one of Europe's most substantial golden eagle populations. A few birds have wandered to Ireland and even nested briefly on Antrim's Fair Head in the 1950s, but the eagle pairs of south-west Scotland no longer rear enough young to make the natural recolonisation of Ireland a realistic early prospect.

The reintroduction of eagles as a Millennium project, deploying chicks removed from wild eyries in the Highlands, Grampians and Mull, followed a decade of groundwork. One big obstacle - the wide use of poison baits by sheep farmers - had been removed by the ban on strychnine in 1991. But did our mountains still have enough natural prey to sustain eagles?

A study for the National Parks and Wildlife Service (NPWS) compared the numbers of grouse, hares and rabbits living in the home ranges of Scottish eagles to the prey available in Donegal, and judged the food supply to be ample. Eagle experts in Scottish Natural Heritage and the Highland Foundation for Wildlife endorsed Glenveagh as an ideal location.

Scottish goodwill and assistance has been vital. It has helped enormously that Lorcan O'Toole, the project's manager and prime mover in the Irish Raptor Study Group, came with the experience of managing reintroductions of the red kite in the Highlands and Central Scotland. He has joined Scottish fieldworkers, voluntary but licensed, in the gentle abduction of eagle chicks, each from a two-chick nest at the age of six or seven weeks - old enough to keep themselves warm and to take food placed on a nest platform in a cage.

The original plan was to launch 75 eaglets into the wild over five years, in the hope of achieving an initial three or four pairs of eagles nesting in the Donegal mountains. On Scottish experience, as many as 80 per cent of fledglings do not last the five years it takes to reach breeding age: there, many immature birds are still shot or poisoned, especially on or near moorland managed by grouse-shooting syndicates.

Donegal lacks the big shooting estates, but it did have apprehensive sheep farmers. A painstaking programme of reassurance, with the help of farmers from Scotland, was an essential part of the project's groundwork. Its success makes it likely that fewer Scottish chicks may be needed to establish the eagles, and as many as seven or eight breeding pairs could result from the initial project.

O'Toole's research for this programme established positive links between local history and the eagles. A local man from Glendowan told him that his grandfather used to tether one of the two young golden eagle chicks to the base of the large stick nest in Glenveagh each year. Once the other chick had fledged and left the nest, the tethered chick was gathered and given to his landlord in lieu of rent. The eagle was then tethered in front of the big house at Ballyconnell, Falcarragh.

Another man, who runs a B&B in Dunlewey, relayed a more recent eagle episode in September. His son was out jogging along the road below Errigal mountain, in training for the Dublin Marathon, when he saw a golden eagle overhead. He watched and followed the soaring eagle as it suddenly went into a steep stoop. He saw the eagle snatch and grab an unsuspecting hooded crow in mid flight and continue onto the slopes of Errigal before eating his quarry. Stories like this boost the eagles' reputation with local sheep farmers, who do not regard the "hoodie" as a friend.

This year ten young birds were released at the end of August, fitted with identifying wing-markers and tiny radio-transmitters. Since hatching they had rarely seen a human form or heard a human voice. They were fed on dead crows and rabbits pushed through sleeves into roomy cages with a mountain view. For the next few months, they will come to food left out on the roof of the cages, mimicking their normal dependency while they are learning to hunt - but from January onwards, they're on their own.

The success of the eagle project has encouraged hopes of reintroducing other vanished species. On some of our northern lakes, 15 artificial nesting platforms have been built by the RSPB, BirdWatch Ireland and the NPWS in the hope of tempting the osprey - the once-abundant fish eagle - to take up residence again. The revival of ospreys has been another conservation success in Scotland, with more than 150 birds fledging each year and migrating to winter in Africa.

Several are seen in Ireland on their spring return.

In his 1999 book, Ireland's Lost Birds, naturalist Gordon D'Arcy presented 11 species once native to the island and extinguished by over-hunting, human intrusion and the loss of woods and wetlands to farming. He has visions of Scots pine forest reinstated in upland Connacht and furnished with goshawks, great spotted woodpecker, and capercaillie, the turkey-like "cock of the wood". In the empty cutaway bogs of the midlands, he sees hope for recreating undisturbed fen and reed-swamp where crane, bittern and marsh harrier might establish themselves again.

The reintroduction of the great spotted woodpecker has the particular enthusiasm of Conor Kelleher, chairman of the Irish Wildlife Trust. As an expert on bats, he thinks that, by drilling holes in Ireland's conifers, the woodpeckers would create new homes for tree-dwelling bat species and make fresh openings for fungi and insects, thus providing an ecological niche missing from our woodlands.

The necessary chicks could, he thinks, come from Wales, but, as with the golden eagle, it would first be vital to find out if the right conditions exist in the older Irish woodlands. Ornithologists in BirdWatch Ireland have wondered if the deforestation of the past might have wiped out the woodpecker's choice of wood-boring beetles, along with other essential insects.

Reintroduction of "lost" birds does not have the unanimous support of ecologists and naturalists, even in Britain, where restoration projects continue apace (restoring the chough to Cornwall and the great bustard to Salisbury Plain are among the latest).

Some fear that such "wildlife gardening" will distance people from a proper relationship with wild nature, encouraging the belief that nature can be safely manipulated and presented for human pleasure. The "engineering" of reintroductions, however well-intentioned and scrupulous, could, they argue, misfire ecologically in a greatly-changed countryside, and detract from conserving the wilder habitats that still exist.

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