I'd imagine that's because the Irish accent is rather different than that of the English and Welsh, though the accent in the north is similar to some Scottish accents supposedly. A lot of Irish folk songs are upbeat you'll find they are either very upbeat and festive or incredibly sombre - that one in particular was a song celebrating the victory of Fiach Mac Aodha Ó Broin over the English Lord Deputy Arthur Grey in the battle of Glenmalure during the Desmond Rebellions.
Less tangentially and more relevantly, that song sounded incredibly familiar, almost as if it was used as part of a soundtrack, I don't think it would have been out of place in a game such as the Final Fantasy series
Spoiler
not something i'd typically listen to, but it's not bad.