How would I say "He is not with us"?
Níl sé linn
Not it have we........I love it!..or it love I...something like that...
níl doesn't mean not, and againn doesn't meant "have we".
It really doesn't make any sense at all to come up with mangled "translations" that are even less "literal" than the actual idiomatic translation.
What's wrong with"It's not ours"?
níl sé againn doesn't mean "It's not ours".
The Irish for "It's not ours" is "Ní linne é*.