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.
https://core.ac.uk/ links to millions of open access research papers, offers datasets for the purpose of text mining and further aggregation services
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
There is also a further open access site https://biblioboard.com/opendissertations/ containing over 800,000 electronic theses and dissertations