Several collections of digitized photos of Ireland on the National Library of Ireland website – see : Digital Photographs.
Their database includes several large collections of photographs, including the Eason postcard collection, images from the Independent Newspaper, Poole collection, and a sellection of Stereo Pair photographs – i.e. in 3D with an appropriate viewer.
Three galleries of photos from the National Library via the Google Cultural Institute :
Witness to War on the Irish War of Independence & Civil War.
Dubliners, The Photographs of J.J. Clarke on James Joyce’s Dublin
Power and Privilege, The Big House in Ireland on Irish Country Houses
