Here is a restored castle of the Desmonds. The display on the family states that the 3rd Earl of Desmond (14th Century) was a noted mathematician! Brendan Lehane has added some details. Gerald Fitzmaurice, variously called the 3rd or 4th Earl of Desmond ( -c1398) inherited the title from his oldest brother in 1359; there seems to have been another elder brother who was 'an idiot' and not allowed to inherit the title. The DNB says 'He is also described as a mathematician and magician', but cites several MS sources. At that time, a mathematician was much more like a magician than a modern mathematician, e.g. like Roger Bacon or Albertus Magnus, so he may not have done anything that we presently would consider mathematics. Indeed, at the siege of Limerick in 1691, some hoped he would reappear from the waters of Lough Air to lead the Irish forces to victory. Can anyone provide more details??
See THIS LINK

An extract from
*The Mathematical Gazetteer of the British Isles*
created by David Singmaster
The original site is at
THIS LINK