I have started a number of different hares in this thread, and I need to go away to do some more homework. Thanks to Brían, Burke, Daystar, and John Moulden for their suggestions and references. There is also some interesting discussion in the IRTRAD-L archives (sorry not to be able to link to these, but the URLs are extremely long - try starting from Listserv Heanet Homepage) ; especially Sean-nos & Scots Gaelic Psalms in the July 1995 archive, and an article by Michael Robinson (from standing-stone) in the July 1996 archive on Gaelic Heterophony.
As a matter of interest the Irish tranlation of The Song of Songs was quoted in an article in Éigse Vol II, signed Colm [? Colm O'Lochlainn], and begins as follows:

Tuggach póg re póig a bhéil dhamh,
Callíon ó shruch Íordáin mé.Repeated after each line
As fearr do chíocha ná an fíon créurag,
Tá cúrrtha ré hunngaibh éifeacht.
As olla t'annim amach séideag:...etc

Colm states that this comes from an entry in Robin Flower's Catalogue of Irish MSS in the British Library, and is from MS.Egerton 167 [not found in BL's online MS catalogue.ME],written (at Damme in Belgium ?) by Lucas Smyth in 1709-10.
The verse seems to be Long Metre, and may represent an attempt towards an Irish language psalter, using what would have been a familiar tune of the time. However, as Brían pointed out, psalm singing was never taken up amongst the Irish Catholic populace.