Without knowing a whole lot about the layout of the whole plumbing system it would be hard to say. All irrigation systems I've seen are tee'd into the water line before it goes into the house and has a checkvalve to prevent contamination of the potable water supply. So, it might be better to run the pipe to Irrigation to the main supply. As for a frost free bib... I would go ahead and cut the drywall if that's going to prevent this from happening again.