Since you did not specify which Gaelic, here are two answersNollaig Chridheil is ScottishIrish would be Nollaig faoi shéan agus faoi shonas duit!
Nollaig Chridheil agus Bliadhna Mhath Ùr
Irish GaelicNollaig shona duit (Happy Christmas to you)Beannachtaí na Nollag (Christmas Greetings)Beannachtaí an tSéasúir (Season's Greetings)Athbhliain faoi mhaise duit (Prosperous New Year)Bliain úr faoi shéan is faoi mhaise duit (Happy New Year to you)Scottish GaelicNollaig chridheil agus bliadhna mhath ur
Since you did not specify which Gaelic, here are two answersNollaig Chridheil is ScottishIrish would be Nollaig faoi shéan agus faoi shonas duit!
In Irish Gaelic:Nollaig shona duit - to one person; Nollaig shona daoibh - to multiple personsIn Scottish Gaelic:Nollaig chridheil!Nollaig shona!
Nollaig shona agus Síocháin Chríost leat (singular) Nollaig shona agus Síocháin Chríost libh (plural)
In Irish its 'nullig huna' (Written as Nollaig shona).
Happy Christmas (Nollaig Shona Dhaoibh) and Blessings of the New Year 2010 (agus Beannachtaí na Bliana Nua 2010)
The correct spelling is "Nollaig faoi shéan agus mhaise dhuit" literally "A prosperous and joyful Christmas" in Irish.
Nollaig shona agus athbhliain faoi mhaise duit!
It sounds like a do-it-yourself translation from someone who doesn't speak Irish.
Nollaig shona dhuit (nnullig hun-a ghitch) = Happy Christmas to you (one person)Nollaig shona dhaoibh (nnullig hun-a gheev) = Happy Christmas to you (plural)In IrishIf you were to say Happy Christmas to just one other person, you would say:Nollaig Shona DhuitHowever, if you were to be addressing the same greeting to two or more persons, you would say:Nollaig Shona DhaoibhThis literally means "You have a Happy Christmas."The most common response to this would be:Nollaig Mhaith ChugatLiterally-"A good Christmas to you."Just as in English, two expressions are often combined to say "Merry Christmas and a Happy New Year" as follows:Nollaig Shona agus Athbhliain faoi Mhaise Dhuit.Le gach dea-ghuí i gcomhair na Nollag agus na h-athbhliana! (With BestThe plural of this would be:Nollaig Shona agus Athbhliain faoi Mhaise Daoibh.For every greeting above, the common response is:Gomba hé duitMeaning: "The same to you."Merry Christmas = Nollaig ShonaWhen speaking to one person: "Nollaig shona duit."When speaking to more than one person: "Nollaig shona daoibh."