I've been using Bookmooch to collect a few of the books on my list of sf and fantasy set in Ireland, and this one was fairly easy to track down.
The premise of the book sounds, frankly, awful. Christa, born in sixth-century Ireland but exiled to 1980s Denver, assembles an all-female heavy metal band (called "Gossamer Axe") to blast open the mystical portals and rescue her girlfriend from the twilight realm where she is imprisoned. To do this she reincarnates her magical harp as an electric guitar.
Yet it's actually rather good. Of course it is rather earnest about paganism, feminism and magic, but the only point where the writing is cringingly embarrassing is in the early sections where Christa is converted from Irish harp music to heavy metal. Apart from that, though, the various romantic and personality plot threads are compelling, and the Irish bits are not overdone. The whole thing is written with a genuine passion which in the end is easier to respect than mock.
It won the 1990 Lambda Literary Award for Lesbian Science Fiction & Fantasy, and was also chosen as one of the "top 5 gay/lesbian/bisexual/transgender genre works of all time" by Gaylaxicon 2000's "The List" panel. (I wonder what the others were?)