I started at the car-park at O185168 on the Glencree to Enniskerry road where the Wicklow Way crosses and heads around Knockree. Follow the Wicklow Way until you get above the forestry line and turn left. This track heads off south around the summit of Prince William's Seat but there's a right turn after about 200 metres. Take this and head up.
I'm not too sure about this being a cairn. The trig point in cemented onto an outcrop. There are very few signs of cairn rubble, just some large stones of the same conglomerate as the outcrop.
The views from here are spectacular, south and west over to Kippure, Tonduff and Djouce, east to the 2 Sugar Loafs, north-north-east to Howth and north down the Glencullen valley.