DIRP is an acronym that stands for "Direction Indicating Rock Pile." It is a term used mainly by hikers referring to a man-made pile of stones (also known as a cairn) built along a trail to mark the route where it may otherwise be hard to follow. Sometimes it may also indicate a destination or location of interest.
Jack: "I have no idea where the trail goes from here."
Jill: "I see a DIRP; the trail is over there."