According to "the docu" Shane had never been to NY before he wrote the song, the title of which was inspired by J.P.Donleavy's novel of the same name, but nothing to do with the book [great, great read]. So, it's about the "failed" Irish immigrants in NY, where Shane had never been.
So really, it's about Shane's alcohol fuelled emotions. (Hat's off to Kirsty too).

RC: good luck with your personal quest.