An Gúm doesn't sell direct to the publlic. They don't seem to have stockist in Belgium, but they do have a listing for the Netherlands:

Brose Wresrestars
Minerebroederstraat 13
3512 GS
Utrecht
tel. +31-30-233 6500

or you could be as well off ordering directly from Irish suppliers. See www.cuplafocal.ie or www.litriocht.com
or contact the book shop of Conradh na Gaeilge in Dublin: tel. +353-1- 478 3814, facs. +353-1- 478 5452, ansiopaleabhar@eircom.net

We have a copy of An Gúm's "Catalóg Cheoil" in the office where I'm working.

Do you think "An Teicheadh go hÉigipt" would be suitable for Christmas? It's available in arrangements for women's choirs or for mixed choirs. This and many of the other songs available were arranged by Éamonn Ó Gallchobhair. He has also arranged a "Duan Nollag" and "Na Leanbhaí i mBeithil" (subject of a Mudcat thread too, but I think only English lyrics are given at the thread) both in four part harmony.

Suantraí na Maighdine (you'll find some lyrics such as Seacht Suailce na Maighdine on Mudcat; I suppose anything about Jesus' mother could be sung at Christmas, or at least the happier songs about her) is another possibility, arranged in three parts by Éamonn Ó Gallchobhair.

There are also other religious songs available that aren't particularly Christmas songs; for instance "A Rí an Domhnaigh" (arranged by Réamonn Ó Frighill), "Bí a Íosa im' Chroíse" & "Dia do Bheatha".

If you want to sing a song and don't have an arrangement, you can make your own or get someone to help you. You could even contact churches or choirs in Ireland; for instance I think the choir of Áras an Ghrianáin in Derry, which sings at an Irish language mass at Nazareth House on Sundays, sings an arrangement of Don Oíche Úd i mBeithil.

For a Belgian-Irish link look for arrangements by Earnán de Regge, from Overmere, who emigrated to Ennis, Co Clare.