a colloquial contraction of '言っても', meaning 'even if (I) say (so)' or 'even if you say...'; used to express concession or contrast.
言ったって、もう手遅れかもしれないよ。
Even if I say so, it might already be too late.
いくら説明したって、言ったって分からない人もいる。
No matter how much you explain, there are people who won't understand even if you tell them.
言ったって仕方ないよ。もう決まったことなんだから。
There's no point in saying (it) now. It's already decided.