Nobody so far has bothered to address 'mike putt's' original question.

Admittedly it was a bloody stupid question, since he didn't think to tell us what country he lives in, or what song he had in mind; but we do now know that that country is Ireland, and that he just wants to print one line of whatever song it is on a t-shirt; and that he intends to credit the writer.

Obviously I am not an expert on Irish copyright law. Unless it is very strange indeed, however, my guess would be that there is no reason whatever why he shouldn't do whatever he wants, unless he is planning to use that one line to promote a political agenda that the writer of the song, whatever it might chance to be, might be opposed to. If that is a possibility, then I'd suggest that he contact whoever wrote the song. Otherwise, I'd suggest that he stop fretting about it and just do it.