Whereas Steeleye Span stuck to existing English, Scottish, & French folk songs (with a few modern instruments and beats), Clannad's specialty was & is traditional Celtic/Gaelic music & original compositions. You are likely to have heard a few of their pieces. They made music for a Robin Hood series in the 80's (where I first heard them), and the theme songs for Harry's Game and Last of the Mohicans. I just realized that this means I have been a fan of their music for well over 20 yrs. They were a well known folk group in Ireland for decades based out of their parent's tavern there. The Clannad singers weren't the most famous in their family. The lead singer, (Moya) Maire Brennan, is Enya's big sister. Those other guys? Those are her brothers. They disbanded as a group some time back (I imagine they still met every Christmas ;) but had a reunion in 2007 that may mean good things for those of us hoping for another album
Here's a few pieces to show their characteristic excellence. You can file this one under world & folk music on the records ;)
Gaelic folk songs
and from the old Robin Hood series (which I remember being quite good in its own right). The original series was finally rereleased on dvd in 2007.
You can hear a clip from "Ancient Forest" & "Lady Marian"