This is the only substantial harp-specific source from any of the Celtic countries:

http://imslp.org/wiki/The_Robert_ap_Huw_Manuscript_%28Anonymous%29
http://www.pbm.com/~lindahl/ap_huw/

There are a lot more melodies that are known to have been played on the harp, but if you find a harp-specific arrangement, it will almost always have been created recently enough to still be copyright and it won't be free.

As an example, there is not a single harp-specific feature in all the known music attributed to Carolan, and it survives only in copies used by flute, recorder and violin players. For another one - some piobaireachd was originally for the harp, but it didn't survive in any harp-playing tradition and we only know about it because pipers and fiddlers preserved it. We can reverse-engineer a few harpy tricks that fit, but that's all.

This is about as substantial as it gets:

https://raretunes.org/patuffa-kennedy-fraser/

and she was mostly making it up as she went along.

A lot of the 18th and 19th century Scottish tune collections, like the Gows and Marshall, suggest the harp as the primary instrument on the title page, but they had the big salon harp in mind, and their arrangements are equally designed for the piano with the melodies designed to fit the violin. This is miles away from anything a folk harpist would want to do.

You're on a hiding to nothing with this. Just arrange tunes you like for the guitar directly and ignore what harpists might have done with them. There is no traditional sound to imitate.