There are numerous traditional versions of the song which, as has been pointed out, is usually known as 'A Sailor's Life' If The Irish Rovers' version doesn't make sense, why not try one of them?
The garbled line is sometimes given as 'On yon green island as we passed by we lost nine more and your sailor boy'(or 'Willie boy'.
The last verse is a 'floater' which appears in many songs.
The tune to which the song is sung is irrelevent, certainly as far as traditional singers are concerned. Quite often, particularly if they learned the song from a ballad sheet, they will use whatever tune that fits. I met a couple of elderly brothers in the West of Ireland who had a couple of dozen songs between them but only used about six tunes.