Just was wondering if double barrelled names are used in Gaeilge. I've always used Mac Cormaic as my Irish surname but my English equivalent surname is a double barrelled name: my mother's maiden name and my father's surname (both of them are Irish) so could I use Mac Thómhais Cormaic (Thompson-McCormick) or would that simply not work?

Is mise le meas,

Antóin

In fact what is Thompson as Gaeilge?? The Thompson side of the family are from Glens of Antrim....

I don't know of any surnames in Irish that are "double barrelled" in the same sense you're describing, but here's a brief description of the practise used in the Gaeltacht:
http://en.wikipedia.org/wiki/Irish_name#Traditional_Gaeltacht_names

Thompson is usually Mac Thomáis.

You can put double barrelled names in Irish, all right

More information -> http://www.nualeargais.ie/gnag/gram.htm

On that page it explains about a particular form of surnames that can be used. It's used in a similar manner to the English practice of calling someone by their surname. For example "Smith", in the sentence "Smith came in earlier with those documents". So it's a way of referring to someone without using their forename.

Someone named Eoghan Ó Ceallaigh could be referred to as "An Ceallach". Muiris Ó Modhráin would be called "An Modhránach". The grammatical rules for forming this name are on that page mentioned above. I believe Mac Thomáis would go to "An Tomásach". How this is relevant to double barrelled names is also explained on that page. Basically, you take the first name in the surname and put it in that form and then leave the second as is. So I believe it would come out as "Antóin Tomásach Mac Cormaic". Of course you don't have to use that, if you like your current way better. I have a double barrelled name myself and I tend to just use the first part in Irish.

I hope that this explanation made some sense, apologies for anything that wasn't clear!

Here's what Rev. Patrick Woulfe (Sloinnte Gaedheal is Gall) says on the matter (p. 27):

"In the case of a double surname in English .. the first surname assumes the -ach termination, thus:"

Patrick Sarsfield O'Donnell (Pádraig Sáirséalach Ó Domhnaill) or
Hugh O'Neill Flanagan (Aodh Niallach Ó Flannagáin).

When two surnames occur together in Irish, they're usually two surnames from the paternal line, or occasionally two names from the maternal line. I haven't heard of using a combination of one maternal surname & one paternal surname as Antóin describes above. Is this a new practise, or am I simply ill-informed?

It is a new practice in English in Ireland, since more women are retaining their names prior to marriage, and both parents wish to see that reflected in the children's surname.

I'm not sure any Irish language usage has settled down yet.