Prince William's Seat and Knocknagun. These are the OS names for two popular hills between Glencullen & Glencree and people often enquire as to the identity of Prince William. It has been suggested that it's named after William, son of George IV, after a royal visit in 1821. Whilst it's possible that there might be a connection with this, in the mind of the official responsible for collecting names at the time, I think it's probable that there is an older origin... Though it's impossible to be 100% definitive, the evidence points to another mix up here by the OS. If we look at maps that predate the OS survey, the situation is different.
For the detailed explanation of this with maps, please refer to the Placenames & Heritage part of my website at http://www.eastwestmapping.ie/placenames-heritage#p10 and scroll to the bottom. In summary, I think the evidence points to the hill labelled by the OS as Prince William's Seat should 'properly' be called Glencullen Mountain and the hill labelled by the OS as Knocknagun should be Cloghnagun with the name (Fitz)Williams Seat for the large granite tor there known as Cloch na gCon. Linkback: https://mountainviews.ie/summit/351/comment/6213/