Translating English “must” into Irish is tricky. Usually, you use forms of the verb caith!/caitheamh, which has many meanings, among them “throw” and “consume”. The forms usually used are the future, which then has a present meaning, and the conditional, which has a past meaning (i.e. “had to”).

Caithfidh mé an obair a chríochnú ‘I must finish the work’

Chaithfinn an obair a chríochnú ‘I had to finish the work’

If a present form is required for formal grammatical reasons, i.e. in a má clause, then use it:

Má chaithim an obair a chríochnú… ‘If I must finish the work…’

Sometimes you do see such constructions as caithim é a dhéanamh ‘I must do it’ (rather than caithfidh mé é a dhéanamh) with present form, or chaith mé é a dhéanamh (rather than chaithfinn é a dhéanamh, or b’éigean dom é a dhéanamh). This usage is in my opinion markedly Munster Irish, and I’d prefer not to use it, unless your aim is to imitate that particular dialect closely, in other features too. (Thus, chaitheas é a dhéanamh feels more natural than chaith mé é a dhéanamh.)

Note the impersonal use of caithfidh sé or just caithfidh in such constructions as caithfidh sé go bhfuil tú sásta, caithfidh go bhfuil tú sásta ‘you must be happy/satisfied’. This means that it can safely be assumed, in the present situation and because of known causes and circumstances, that you are happy. If you say caithfidh tú bheith sásta, that sounds as if there was a dictator telling you that you must show a happy face.

Obviously, there are other ways to translate “must”. The direct translation of the English “I have to do it”, tá agam é a dhéanamh, is used by native speakers, although tá orm é a dhéanamh might fit better in, noting that duties and responsibilities are in Irish usually “on” (ar) you.