What are these wires I found in my yard?
The wires do indeed go to the sprinkler system. There turned out to be a 4th white wire I hadn't yet uncovered. The two sets provide the current to open control valves for the two zones on the other side of the driveway.

I finally got input from a professional sprinkler repair company. They were baffled at first, because the pipes appear to go all the way around the house. The sprinkler guys finally decided that those had to be stubbed extensions for adding a zone to the backyard. Additionally, the wires turn under the drive at a different point than the actual water supply pipe.

Finally, I didn't detect the problem because there was another long standing problem with the zone they supplied that prevented it from getting water pressure.