A hymn tune? That's in interesting thought.

It does remind me of the Welsh tune, Aberystwyth, which you can hear playing here:

http://www.hymntime.com/tch/htm/h/o/holylord.htm

As for the mysterious Mr. MacIntyre, I think it's interesting that the sort of people who would sing a song like this have voted for rhythm over reason. By that I mean, that they have excised the t' from 'somebody shouted t' MacIntyre' in order to have the song scan better.

Also, to be able to shout back at the singer, albeit in a jolly way.