"Oh, and that other thing, them two hitters you asked after, they good with it" - Shamrock
He's not referring to hitters for Avon. You're correct that Stringer was selling out Avon (to clarify, I don't believe Stringer actually technically sold information to Bunny, just rats Avon out instead).
Earlier, Stringer wants Slim Charles to assassinate Clay Davis (Downtown Clay Davis) but Slim and Avon push back and say no.
This doesn't stop Stringer, who then seems to have reached out for someone to do it. The two hitters Shamrock is referring to are two guys who will assassinate Clay Davis.
Edit: Major spoilers following - hover over the spoiler below to see what I'm saying
When Stringer is killed, I assume they abandon the hit because killing a State Senator is obviously a huge deal and with String dead there's nobody to threaten them to complete the job or likely even pay them