I think, SatharnPHL, you should explain why. Clearly it should be "m'ainm" rather than "mo ainm". But is there a grammatical reason why one could not say "is Pól m'ainm"? Also, it is necessary to explain the grammatical structure of "Pól is ainm dom": as I understand it, this is literally "[it is] Pól [that] is my name", as one does not usually put words before the Irish "is".
I could explain why you're getting it wrong if you could explain why you think is Pól m'ainm is correct.
But this isn't a theoretical example, it's a practical one. Irish speakers say Pól is ainm dom where English speakers say "My name is Paul". Even if "is pól mo ainm" was grammatically valid, it wouldn't "work", because that's not what Irish speakers say in a situation where English speakers say "My name is Paul", just as buí doesn't work when an English speaker says "blue".
The defacto standard dictionaries, Ó Dónaill's Foclóir Gaeilge-Béarla, and Foras na Gaeilge's New English Irish-Dictionary, both prefer Dia duit.
de Bhaldraithe's older English-Irish Dictionary does give Dia dhuit, but there is no question that Dia duit is now the "standard" way to spell this phrase, even if you pronounce it as though it was Dia dhuit.