Birthday/s: Co-L`a-Breith (Coh-laa-brey, Literally means Combined day birth, which does make sense)
Happy Birthday: Meall Do Naidheachd (myel doh Nay-achd, Literally means good is your news).
Christmas: Nollaig
Merry Christmas: Nollaig Chridheal (Literally means a heartfull christmas)
New Years: Bliadhna (blee-an-ah) Ur (oor).
Easter: A ch`aisg
Halloween: Samhain
(please note, November has a very similar name. An t-Samhain, which is prnounced An Tamh-ain (the mh almost sounds like V).