To say what you would like, use "Ba mhaith" followed by a form of the preposition "le" that already contains the person: liom (I), leat (you), leis (he). The thing you want comes after it. To ask someone, the question form is "Ar mhaith leat...?".
Examples
- Ba mhaith liom cupán tae, le do thoil.
- I would like a cup of tea, please.
- Ar mhaith leat uisce?
- Would you like water?
- Ba mhaith liom an bille, le do thoil.
- I would like the bill, please.
The mistake to avoid
Asking "An maith leat...?" instead of "Ar mhaith leat...?" — the first one means "do you like it (in general)?", not "would you like one now?".
Where this fits in
Food & ordering, Irish Gaelic A1: Order at a café or restaurant, ask for the bill, and talk about basic food and drink.