He's very good to his mother
Coded warning. Said of a man who looks saintly on paper and is absolute trouble in practice.
"Ah sure he's very good to his mother." (meaning: complete sociopath)

Shall I translate it into posh English for you, sham?
Good day to you. Here it is in posh English.
Oh, he is most attentive to his mother. (Meaning: a thoroughgoing menace.)
Questions
What does he's very good to his mother mean in Ireland?
Coded warning. Said of a man who looks saintly on paper and is absolute trouble in practice.
How do you use he's very good to his mother in a sentence?
“"Ah sure he's very good to his mother." (meaning: complete sociopath)”
Updated
On the list: Irish slang phrases