Sue, in Ireland at any rate, a haunting traditional fiddle air/tune/lament would almost invariably be the tune of a song.

Having said that there's a song called An Mhaidhdean Mhara (literally, The Mermaid), from the Gweedore district of Donegal, which I think would fit your purposes admirably, and which exists in two very fine commercial recordings that I know of:

Amhrain ar an Seán-Nos. RTÉ. CD 185. Sung by Aine Uí Laoi        

World Library of Folk and Primitive Music Vol 11-Ireland. Rounder CD 1742. Sung by Cití Ní Ghallchóir (Kitty Gallagher)        

I've no idea whether the RTÉ CD is still available, and I'm fairly certain the Rounder has been discontinued. However, if you follow this link http://theworldjukebox.wordpress.com/page/3/ you can listen to an uploaded copy of the vinyl LP from which the Rounder was re-issued.

The Lp tracks aren't sectioned, but An Mhaidhdean Mhara is on side A, track No 8. It is preceded by a pair of dandling songs; Dance to Your Daddy and Cucanandy, both sung by Elizabeth Cronin.