Haha was just about to pull you up on that one!
Surnames are an interesting one actually. Irish surnames are quite distinctive for some reason, even McCarthy and McGeady can be easily distinguishable in another Celtic nation. I suppose they are the best indirect indicator of Irish descent, the only other indicator can be the player himself.
Grannies and mothers who don't carry their second name and whose maiden name isn't reflected in their offspring's name. Jon Walters and Joe Mason are two similarly recent instances. Or, in the words of Tony Cascarino, "of course I qualified to play for Ireland, my mother's name is Teresa O'Malley!"
There are plenty of instances of South-American footballers of Irish descent, but they often seem to be too many generations out. Diego Forlan, Jose Luis Brown, Alfredo Di Stefano and Fabian O'Neill are three examples off the top of my head.
EDIT:
http://www.irlandeses.org/imsla0803.htm
Bit of research done.