Selecting water pipe for bypass

It looks to me like it's still the sprinkler pipe, they're attached at the top? So that line that's covered in ice is probably the supply that comes from inside your house, and the line going down goes out to the sprinkler system, and there is a backflow preventer at the very top where they connect.

When you shut it off, there was still water in the line going outside, and it's likely that it burst. The pattern of ice looks like there was water flowing from the backflow preventer. There's a lot of water there though, I don't think the water in the lines would be enough to make that much ice. You should double-check that you shut that line off. You should be able to see where that comes into the house, and there should be a valve on it. If that line isn't shut off, and has also burst, then as soon as the ice melts it will start leaking/spraying again, and as it gets cold freeze again, etc.

I'd say you have another shutoff that you missed (eg, did you shut off the sillcock (outside tap) next to the sprinkler instead of this line?). In the spring, you'll have to replace part of that pipe or the backflow preventer, depending on what broke.

If this pipe is just above ground level and is spewing clean water out onto the ground (or is otherwise unchecked), it is more than likely coming from a sump. This sump could be from a drain tile or the A/C drain. If you have access to the basement, this can easily be determined by tracing the pipe back from where it goes through the wall/structure, unless it is all behind sheetrock.

(NOTE: I didn't see Tester101 and Fiasco Labs comments prior to writing this. I'm basically agreeing with them.)