First, the standard (long form) New Year's greeting is "Athbhliain faoi Shéan agus faoi Mhaise Duit/Daoibh" (A Prosperous and Beautiful New Year to You/ Y'all).
the new millennium = an mhílaois nua
the second millennium = an dara mílaois
the year 2000 = an bhliain 2000 (dhá mhíle)
If you want to welcome it, you could say:
Fáilte roimh an M(h)ílaois Nua ("h" in Ulster)
Fáilte roimh an Dara Mílaois
Fáilte roimh an mBliain 2000 ("an Bhliain" in Ulster)
"Stair an Fhocail"
http://www.eskimo.com/~donncha/stair3.html
