This is a hornpipe in the key of D.
The arrangement in O'Neill's Music of Ireland (revised by Myles Krassen) contains F# throughout the whole of the tune, but it is often played with F naturals in the second part.
Tony's arrangement is the older style, with F# throughout.
Hornpipes are played considerably more slowly than reels.
Chords for accompaniment are D, G, A and C.