Lots of ways but I think the nicest is "Mo grá thú" pronounced Muh graw who. There are many terms of endearment such as
Is Tusa (Iss Tuh-sah) (You are) ...
Mo Rún (My secret) (Muh Roon)
Mo Stór (My treasure) (muh store)
Mo Chuisle (My pulse) (Muh coosh-la) hence the song Macushla
Mo Chroí (My heart) (muh-cree) hence "Mother Macree"
to mention but a few.