On the Waterboys album it's F G Am C but who says you can't play it any key you want?

Whichever key you play it in the chord sequence goes IV V VIm I (it misses out the I chord in the bits before the fiddle bit)

In D it indeed starts with a G chord - in F# it would start with a B etc

Pentatonic VIm run when it hits the VIm chord to my ears in the solo

I play bass on the tune and do the same as the fiddle only a couple of octaves down

Lots of embellishments on the fiddle part but Sarah's post is good stuff for the 'tune'