The band that does "Tir na Nog" (from the '70s) is on the tip of my tongue...I am on it!

Are you mainly interested in Irish-based mythology? There are also plenty of songs about old folk heroes that are English ballads, or Scottish...Bonny Prince Charlie inspired quite a few in Ireland and Scotland...

Thomas the Rhymer by Steeleye Span comes to mind...

Standing Stones (recorded by Loreena McKennitt) tells a tragic love story set near a mythical lover's stone in the Orkney isles...