Most arrangements I've heard tend to be disappointing from a folk music point of view.

There is the volume of Britten folk song arrangements for voice and guitar, with accompaniments varying from very simple chordal accompaniment to some rather more startling ones (Bonny At Morn!).

There have been quite a few arrangements of tunes - David Russell has an album of celtic music (Message of the Sea). And Gerald Garcia has arranged some - Carolan and others IIRC.

Scott Tennant of the LAGQ has an arrangement of Wild Mountain Thyme, but I'd say it's fairly straightforward - pleasant but not not wildly exciting.

There is an album Chords & Thyme by Edward Flower of English Folksong for guitar. I haven't listened to it for a long time and I can't remember what the arrangements were like.

I'll have a look and see what else I've got.

Mick