The Clancy Bros and Tommy Makem did record the Mingulay Boat Song but this isn't their recording (I remember Tommy Makem's voice on the recording; I think the band took turns on the verses). It's closer to the Dubliners. Must be some lesser known group. Maybe "the Irish Brothers" a different group of Clancys from Co Tyrone rather than Co Tipperary? That's what I remember but when I looked up The Irish Rovers I found references to a band based in Toronto, Canada

Their recording of the Star of the County Down sounded to me like the group on your recording, singer with a similar voice. As you say, the recording is probably speeded up. The recording of Mingulay boat song sounds different to me, deeper, no prominent banjo, but does have the a capella harmony singing towards the end of the song.
https://www.youtube.com/watch?v=xy0OcjkVdGM What do you think?