I don't have a lot of information on Thomas Walsh except I've been told he was/is a guitarist from Dublin. I know I guy called Tom Walsh who plays guitar but it is not (as far as I know) the same guy. I'm fairly sure he wrote the tune without words. Some sets of words have been added since but not by him. So I think he is the composer of the tune. I don't have any huge evidence for any of this information, although I'm fairly sure its accurate. PRS (or its Irish sister organisation IMRO) should be able to advise you on this.