The use of the key of A was induced neither thru Scot's nor Scotch
influence. I learned Mountains of Pomeroy in A and preferred to stay
in the same key when changing tempo. I have seen the tune printed in
the keys of D, G and C.
According to Folksongs & Ballads Popular in Ireland Vol.2, Jim is right
about the Wexford connection:
"A song about the Wexford rising of 1798. John Kelly was a
merchant's son from Killanne, Co. Wexford. After fighting at
New Ross and Wexford he was captured by the English and
hanged. The words are by P.J.McCall, the author of Follow me
up to Carlow and Boulavogue".
I was told by someone, perhaps Noel Hill or Tony Crehan, that both
tunes were very popular in Tyrone.