IONA is excellent indeed, I have Beyond These Shores, Journey Into the Morn and Open Sky, and enjoy all them, this is Celtic-influenced prog at its best. Here's the link to their website:
http://www.iona.uk.com/newindex/homepage.html
Another great band in the same Celtic vein is MOSTLY AUTUMN. They have three albums released to date - For All We Shared, The Spirit of Autumn Past and The Last Bright Light -, and also and EP (Prints in the Stone). Sound samples available at this link:
http://www.mostly-autumn.com/frameindex.html