Rapaire's claim that Dominic Behan wrote it seems to me pretty unlikely. Undoubtedly Dominic Behan used his name for the copyright when he recorded it, but this is common practise among folksingers in order to get royalties off folk/public domain/uncertain authorship material. he did the same for the Foggy Dew and many other Irish songs, but that is not the same as claiming authorship. He probably tweaked the words, as most folksingers do.
Just for interest, here is the chorus as Dominic recorded it on his "Rebellion" LP

So I'm off to Dublin in the green in the green
Where the the helmets glitter in the sun
Where the rifles crash and the thunders crash
To the echo of the Thompson guns.

The song goes back a long way, and didnt used to have any IRA connections. The "Merry Ploughboy has been knocking around in England for a long time, with no Irish connections, as a "off to join the army" sort of song. Whether it started in Britain or Ireland I've no idea, but the Dublin/IRA currently popular version is certainly recent, though I would think pre-Dominic Behan.