Yes I've heard stuff from all three bands you mentioned. Clannad are unique in combining the old celtic with the modern synth sound - it's a pity Enya is trying to copy them. She would be better off getting her own sound.
Four Men and a Dog are supposed to be excellent in concert. Friends of mine have seen them and are raving about them. I've heard stuff on tape but I really can't remember what I thought.
I am a big fan of the singer out of Silly Wizard. Their playing leaves much to be desired - too much "Hey-ho and away we go" on the box (accordian). The singer, however is brilliant - his name escapes me, but his voice sends me off into a trance. The only two songs I have of him are "If I were a Blackbird" and "The Fisherman Wife's Lament". I am open to suggestions of other tracks.
If you ever get a chance, check out the band More Power to your Elbow. I think they're from the Free State somewhere. Funky rock traditional - excellent!
Slan go foill