As most people learning Irish know, the future of the verb caith!/caitheamh is used in the sense of “must”, and the conditional moood in the sense of “had to”. Many ask me whether it is possible to use the present of the verb in this sense at all.
I answer: yes it is, if it is syntactically required, i.e. after má ‘if’. which is followed by present tense if there is a future form in the main clause: Má chaitheann tú an rud áirithe seo a dhéanamh, caithfidh mé é a dhéanamh chomh maith ‘If you must do this particular thing, I must do it too’.
It is nowadays common practice to use caith!/caitheamh in the sense ‘must’ in its all tenses. It is my impression that this is a specifically Munster usage, or was to start with. If you say Chaith mé é a dhéanamh ‘I had to do it’, I can’t dismiss it as out and out wrong, but I wouldn’t use it myself. (And of course, if I was trying to imitate Kerry Irish for artistic purposes, I would say Chaitheas é a dhéanamh.) My preferred practice would be to say B’éigean dom é a dhéanamh.
In fact, I would not object to Bhí agam é a dhéanamh, although obviously a loan-translation from English (‘I had to do it’), because this is an established structure in those dialects I am most familiar with. However, it does clash somewhat with the native logic of prepositional usage, because in the sense of obligation, ar rather than ag is the usual preposition – bhí orm é a dhéanamh should thus be preferred.
Note that the impersonal caithfidh sé is preferred when we are speaking of things that it seems must be true: caithfidh sé go bhfuil tú sásta means ‘you must be happy’ in the sense that external circumstances suggest you are happy. (My anonymous commenter says that it feels more natural to omit the sé, i.e. caithfidh go bhfuil tú sásta, and while I am not sure about “more natural”, I do agree that in this particular expression the subject pronoun can be omitted and often is, and it is absolutely OK to omit it.) If you say caithfidh tú bheith sásta, the sense conveyed is that you must be happy because there is some sort of obligation upon you: caithfidh tú bheith sásta nó cuirfidh mé as oidhreacht thú! ‘you must be happy or I’ll disinherit you!’ (In this sense though, I guess it would be more idiomatic Irish to say caithfidh tú cuma shásta a chur ort nó cuirfidh mé as oidhreacht thú ‘you must put on a happy face [in Irish, ‘you must put a happy expression on yourself’] or I’ll disinherit you’).