A colloquial contraction of 'なければ', meaning 'if (something) is not present' or 'unless (one) ...'; commonly used in spoken Japanese.
金が無けりゃ、旅行なんて行けないよ。
If you don't have money, you can't go on a trip.
時間が無けりゃ、手伝えないんだ。
If I don't have time, I can't help you.
代わりが無けりゃ、この案で進めるしかない。
If there's no alternative, we have no choice but to proceed with this plan.