I have a question about two verb forms. One is the past subjunctive and the other is the non-2nd person imperatives. Are these forms still used in everyday speech? If not, what is substituted for them?

Concerning the past subjunctive. I have seen this covered in some books, but one grammar book I have completely skips over it. They don't say why, but I assume because it isn't used much.

The non-2nd person imperatives, that is the 1st person plural, and the 3rd person singular and plural (let us... let him/her/it, and let them ...), are listed in modern books that I have, but are they limited to the usual phrases like "let's go" and "let him be" or the Beatles in Irish "Let it be"? Or even the 1st person singular (let me be)?

Go raibh maith agat!

my understanding is that the imperatives are still used, like we'd say in english, "let's go" that would be 1p.pl. imperative, etc.

I found this on past subjunctive: http://en.wikibooks.org/wiki/Irish/Verbs#Past_Subjunctive

I've learnt that the past subjunctive (whose name is quite strange since it's completely different in form an meaning from the present subjunctive) is mainly used, in older speakers' speech, instead of the conditional after dá and mura/muna etc. I was told it had the same meaning as the conditional.

Maidir leis an 3ú pearsa ordaitheach, ná déan dearmad le frasaí mar:
"Ná bíodh eagla ort!" (Don't be afraid)
"Ná bíodh fearg oraibh liom!" (Don't be angry with me)
agus:
"bíodh an lá fliuch nó tirim" (be the day wet or dry, i.e. no matter if the day is wet or dry)

Lars