The summit of Dome sports the Owl Pinnacle which has two short, but worthy bolted routes on the north face.
From the Southern Wonderland parking area (the dirt lot just east of Barker Dam Parking area) head east past the gate on an old dirt road. After about 80 yards, take the left fork toward some ruins (Uncle Willies Health Food Store). From the ruins, head left (west) under the trees into a wash (the Southern Wash). Soon the wash makes a sharp right-hand turn and heads north. Proceed north along the Southern Wash for about 0.6 mile where it begins to open up into a broader valley (the Wonderland Valley). Now head slightly to the left and then proceed through a narrow "S" spot on the main trail. You should now be able to see the Astro Domes to the northwest (leftish); Foolproof Tower is located about 100 yards directly west (left).
Continue straight on a fairly well marked trail that proceeds pretty much directly north (initially, low rocks will be immediately on your right). Stay on this north bound course for 0.6 mile, occasionally tending a bit right (exiting washes that run northwest) to stay on course. Eventually you will pass Surprise Rock and then Freak Brothers Dome, seen a short distance to your right. Photo Just past Freak Brothers Dome is a large free standing pillar on your right (east). This pillar is The Weenie Photo (1.2 miles from car). To your left (west) Punk Rock sits high on a small hill.
From the Weenie, continue on a north-northwest course in the broad and flat Wonderland Valley for another 750 yards (1.6+ miles from car). You will come to a point where the main valley and wash veers slightly to the left (northwest) and a low rocky ridge (the Hogsback) is located almost directly ahead. At this point the distinctive Diarrhea Dome (with the large boulder on top: Owl Pinnacle) will be visible to the northeast.
As you pass through some scrub oak along the wash, just before the rocky ridge "Hogsback", a wash will be seen coming in from your right. From here, head right (east) along this wash which is almost immediately blocked by large boulders. See Photo below of this spot
Crawl under the boulders and follow the wash beyond for about 25 yards. Now, head left (north) in a small brushy side canyon. Soon the climber trail ends and you angle right and up along slabs and pass just right of a large boulder. From here, angle slightly right across slabs and you will see Diarrhea Dome's south face a short distance to your northeast. See above Photo of the view here Proceed across slabs and then drop right (east) into a small canyon that runs to the north).
Bighorn Mating Grotto Directions: If you are heading to Bighorn Mating Grotto, follow this small wash/canyon north past the west end of Diarrhea Dome until you are in a rocky area below a notch on the west side of the formation. A tree is located in one of several gullies here. Scramble up the gully and over the notch directly into The Grotto.
South Face of Bighorn Dome Directions: After dropping into the small canyon/gully, head north in it a short distance until you are near the southwest corner of the most western part of Diarrhea Dome, then scramble right and up along the base of the formation.
(1.8+ miles from car to base of routes).
Modified from: Joshua Tree Classic Climbs and Boulders (c) 2011; and Joshua Tree Rock Climbing, Central Joshua Tree , copyright 2011 by Randy Vogel
Mountain Project's determination of some of the classic, most popular, highest rated routes for Diarrhea Dome: