This melody is fascinating.

It's worth noting that musical accompaniment was limited during the first millenium of Christianity. The organ was totally banned, due to its ancient connections with brothels. (Just like jazz, eh?)

In my own monastery, there is no organ; everything is sung a cappella. I know what it's like to get thirty men to harmonize!!!

Also, I couldn't tell from the clip but I wondered if the congregation was repeating what the cantor/leader was singing. That would be the usual method of song for Congregational churches in colonial New England. "Amazing Grace" can still be heard like this.

As regards the use of Gaelic by African slaves, during that same period, Cromwell's troops would routinely seize Irish men, women and youth right off the streets and roads and send them to Barbados as slaves. Almost the entire body of the Catholic clergy of Ireland were shipped off in this way at one point. I've heard it that this accounts for the Barbadian(?) accent.