The first and primary site for Irish material would be http://rian.ie - this site hosts theses from all seven Irish universities and is pretty easy to use.
http://www.opendoar.org/ contains a whole host of open access repositories from around the world. You can access a search box by clicking on 'search repository contents'; alternatively, you could access the 'list of repositories' and focus on those from the United Kingdom, or another country of your choice.
http://www.dart-europe.eu/basic-search.php - a partnership of research libraries and library consortia which provides access to European Electronic Theses and Dissertations.
There's a separate site where you can access UK theses exclusively, which is http://ethos.bl.uk/Home.do