Of the Planxty voices, I still prefer Andy Irvine, closely followed by Paul Brady in traditional mode - their duo album was one of my favourites back then. Paul Brady's "Welcome Here, Kind Stranger" also a favourite from the same period. Also "The Missing Liberty Tapes" is a live rendering of material from that album.

In June Andy Irvine proposes to celebrate his seventieth (!) birthday with a bunch of his former colleagues [Sweeney's Men, Mozaik, Andy & Paul, LAPD (Liam O'Flynn, Andy, Donal Lunny, Paddy Glackin)]in a couple of concerts in Dublin http://www.andyirvine.com/calendar.html I haven't heard ALL the albums he has been connected with, but I reckon any one of them could be recommended.

Andy Irvine was also one of the band who backed the Silly Sisters (Maddy Prior and June Tabor) on tour and on record (1976). Recommended.

Also in that band were Nic Jones (already recommended above):
Tony Hall, an extremely talented and relaxed player of the melodeon from East Anglia, whose albums of tunes and songs are worth a listen (Fieldvole Music, 1977):
Johnny Moynihan, who for better or worse introduced the bouzouki to Irish music, and an associate of Andy Irvine's in Sweeney's Men - I wish there was an album from him!
and Liam O'Flynn (many solo albums - one of the most lyrical of the uilleann pipers - as well as his traditional material, Shaun Davey wrote several orchestral suites to feature Liam's piping "The Brendan Voyage" still a favourite, though "The Pilgrim" and "Granuaille" also recommended.

Suibhne mentioned Strawhead and the redoubtable Gregg Butler. Apart from the thirty-eight years of Strawhead material and a twenty-year-old album for the last Preston Guild, as far as I know Gregg's only other recording credit is on the Shirley Collins/Albion Country Band "No Roses" album which I don't think anyone has mentioned yet - highly recommended.

Neil Wayne produced a series of great albums on the Free Reed label in the seventies, including Peter Bellamy's "Transports" which you have no doubt come across. Many of those albums are classics of the period, now once more available in re-mastered form. "Plain Capers" features John Kirkpatrick and others on an album of Morris tunes. Recommended.

I could go on - but I'm nursing a cold and I should go back to bed.
Good luck with the search and good listening!

Ross