Nollaig shona dhuit agus athbhliain faoi shean is faoi mhaise (pron: nullig hunna gwitch oggus a-vleeyan fwee hyann iss fwee washa) = Happy Christmas to you and a prosperous and (umm...literally "decorated") wealthy new year.

Beannachtai na Nollaig dhuit is what you say, Nollaig shona is what you put on cards and decorations.