What is the etymology of the Sicilian expression "amuninni" / "amunì"? I am assuming that the -nni suffix corresponds to Italian -ne as in "andarsene". Is "amunì" a 2. plural imperative of a verb (annari?) If it is, which verb?
I don't know about the etymology of "amuninni" / "amunì", but I can tell you that they are the plural exhortative of the verb "iri".
I found this page with conjugations of some Sicilian irregular verbs, where there is no imperative nor exhortative, and the page of the Sicilian Wikipedia for the verb jiri/giri/ghiri/iri where there is the plural imperative "jemu". However there is no exhortative, I am afraid you have just to believe me.
Googling "amunì esortativo" I found this piece of art in which description you can find a reference to the exhortative purpose of of the expression.
Don't bother too much about the different spellings.
In Sicily each city has it's own slightly different version of Sicilian. For example people from where I come from would use indiscriminately iri/jiri (I think they are pronunced in the same way) or ghiri, but never giri.
And they would say "nantri emu" or at most "nantri iemu", instead of the Palermitan "nuatri iamu" which you find in the link to the irregular verbs.