In Irish you have - an aimsir chaite (simple past), - an aimsir ghnáthchaite (imperfect) and you can use bhí ag (past continuous) but how do you express the present perfect (I have just done it) and the past perfect (I had done it) ? and I would have done it if..... I could have done it if......? if only I had known? etc - this is the sentence that I don't know how to put into Irish - he had seen nothing that he could shoot at.

when the meaning is clear, you can use the normal past.
If you really need to emphasize the perfect aspect, then you can use "bhí sé déanta agam" (it was done by me), bheadh sé déanta agam dá (is would be done by me, if), dá mbíodh's agam (if I had known, although I heard there were no difference between dá mbeadh and dá mbíodh...)...
But expressiong the perfect aspect with the verb to be + the verbal adjective doesn't work with all verbs (and anyway some verbs have no verbal adjective).

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

Go raibh maith agat

You can also (for the ‘I have just done it’ structure) use the construction that has entered Hibernian English as ‘I’m (just) after doing it’: tá mé (díreach) i ndiaidh/tar éis/théis é a dhéanamh—which, since it uses tá is easy to put into different tenses.

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Not a native speaker.

Always wait for at least three people to agree on a translation, especially if it’s for something permanent.

My translations are usually GU (Ulster Irish), unless CO (Standard Orthography) is requested.

Dia daoibh ar maidin - ça y est, j'ai trouvé in an old second-hand copy of 'New Irish Grammar' by the Christian Brothers that I've managed to get hold of - page 93 (Compound Tenses)
Perfect Tenses:- Bí (tá, beidh, etc.) + verbal noun or verbal adjective:-
Tá mé i ndiaidh an bád a dhíol (I have sold the boat)
Bhí mé tar éis an teach a ghlanadh (I had cleaned the house)
Tá an bád díolta (the boat is sold)
but the question is when do you use a verbal noun and when do you use a verbal adjective, and what is the nuance between them?

Tá mé i ndiaidh an bád a dhíol = je viens de vendre le bâteau
Tá an bád díolta = le bâteau est (déjà) vendu

More or less, anyway.

The i ndiaidh/tar éis construction is an active construction: I have just done it. The adjectival construction is passive: it is sold.

_________________
Not a native speaker.

Always wait for at least three people to agree on a translation, especially if it’s for something permanent.

My translations are usually GU (Ulster Irish), unless CO (Standard Orthography) is requested.

Go raibh maith agat