If you are in Dublin check out the following stores for old British comics:
Sub City Comics . They have two stores in the city centre, and have large back issue stock.
There is a second hand book and record store that often has very obscure music magazines, comics and old annuals called The Secret Book and Record Store (it's also known as Freebird Records)
There are other very good comic shops in Dublin; The Big Bang , Forbidden Planet and Dublin City Comics, but they mainly stock new comics and US back issues.