Since Ireland was completely covered by ice sheets during the most recent Ice Age, all native plant and animal life now extant originated from the natural migration of species, chiefly from other parts of Europe and especially from Britain. Early sea inundation of the land bridge connecting Ireland and Britain prevented further migration after 6000 BC . Although many species have subsequently been introduced, Ireland has a much narrower range of flora and fauna than Britain. Forest is the natural dominant vegetation, but the total forest area is now only 5% of the total area, and most of that remains because of the state afforestation program. The natural forest cover was chiefly mixed sessile oak woodland with ash, wych elm, birch, and yew. Pine was dominant on poorer soils, with rowan and birch. Beech and lime are notable natural absentees that thrive when introduced.
The fauna of Ireland is basically similar to that of Britain, but there are some notable gaps. Among those absent are weasel, polecat, wildcat, most shrews, moles, water voles, roe deer, snakes, and common toads. There are also fewer bird and insect species. Some introduced animals, such as the rabbit and brown rat, have been very successful. Ireland has some species not native to Britain, such as the spotted slug and certain species of wood lice. Ireland's isolation has made it notably free from plant and animal diseases. Among the common domestic animals, Ireland is particularly noted for its fine horses, dogs, and cattle. The Connemara pony, Irish wolfhound, Kerry blue terrier, and several types of cattle and sheep are recognized as distinct breeds.