A hantle (hand-full) o sources o guid Celtic kirk music:

Buiks:
"Songs of Celtic Christianity" by Dennis & Paula Doyle
(ISBN 1-886314-00-4) Published by Incarnation Music, P.O. Box 1061, Glendale, CA 91209, phone: (818)956-1311, e-mail: carolan@aol.com
(Traditional hymns & a few originals, arrangements are very simple & there are two cassettes of the material available for those who learn better by ear. This is a nice, accessible resource, but I found myself wishing for more ornamentation, complex harmony, and that old "minor modal" Celtic sound--things that would probably make the music LESS accessible to the average American!

"Songs and Hymns of Scotland" by L. MacBean
(ISBN 0-912951-51-6) Published by Scotpress, P.O. Box 397, Bruceton Mills, WV 26525. This is a 1988 reprint of Eneas MacKay's "The Songs & Hymns of the Gael", first published in Stirling in 1900. Songs are in Scottish Gaelic with florid Victorian "singable" translations.

The Iona Community (an intentional ecumenical faith community based on Scotland's Isle of Iona) & its publishing arm, "Wild Goose Press", have created a wide range of ecumenical worship materials based on Celtic styles & sources. The quality of the material is quite good. I'm not sure of their website, but you can probably track them down through a search.

Recordings:
Sean O' Riada, Irish Traditional Music Revivalist & Composer, set many traditional Irish prayers to music and composed a number of Irish-language masses. His setting of "Ag Criost an Siol" (To Christ the Seed) is sung by Maria Doyle Kennedy on the 1997 Work/Sony Wonder compilation, "The Planet Sleeps" (OK 67772). It's a beautiful song that would suit your sermon/service theme quite nicely.

"Lasair Dhe/Flame of God: A Celebration of Gaelic Spiritual Music", (SKYECD19, copyright 2001 Macmeannma) recorded by Cliar with guests Kenna Campbell & Donnie Murdo MacLeod & a 200-voice Gaelic choir, is another excellent recording. The work won several major awards in Scotland. With the exception of field recordings (available from the School of Scottish Studies), no other recording matches this one for cultural authenticity. Sheet music might be available; contact the publisher/copyright holder.

Ah've offerit some Scottish Hebridean resources aboon, an that micht seem wrang-heidit, but Ah've haird it said that the folk o Ireland & the folk o the Hebrides are claise cousins frae common stock, wi muckle shared atween 'em. Mony o the priests sairvin Hebridean kirks cam frae Ireland theirsels, an the kirk traditions o Scottish Hielan Catholics & Irish Catholics were claise indeed, includin their choice o hymns. There are ither resources ayont the anes Ah've screevit, o course, but these are the anes Ah've haed a chance tae luik/listen ower, an Ah've field-testit 'em a wee bit tae be sure.

An Beannachd Oirbh/Blessings,

--Cuilionn