A lovely tune, though I see it's called "Boyne Water' at a faster tempo.
My only contribution would be that the intro starts on notes C and B on a high string
and then plays the A and F of the F chord; the A then descends scalewise.

The opening chords seem to me to be F, C Gm Am Dm.

As regards tuning, there is a colourful chord in the intro which precedes the tonic which might give a clue.
You might be tempted to think it's in a minor tuning, but standard a possibility.