From the trailhead, follow the Eldorado Canyon Trail past the switchbacks until you reach the sign for the Rincon Cutoff Trail. Take the Rincon Cutoff Trail. Once on the Rincon Cutoff Trail, look for a faint trail that leaves the Rincon Cutoff Trail to the left (about 500 feet or so up the Rincon Cutoff Trail, at a point where the Rincon Cutoff Trail starts to head downhill). Follow this new trail a short distance to a boulderfield, then head up the boulderfield to the saddle on the ridge between the Cadillac Crag and Rincon Wall.
From the saddle, traverse south below the ridge until you get an opportunity to perform some class 3 moves to get onto the ridge proper.
This is what you'll see to the north if you climb to the top of the ridge at the point shown above. Shirttail Peak is southeast from here.
Scramble south along the east side the ridge until you find an opportunity to get off the ridge and head toward Shirttail Peak. Reaching the summit of Shirttail Peak requires a few class 2+ moves.