Morphology: originally from mynydd+is+llwyn
Meaning "the mountain below the grove", the hill has given its name to so much that is important in the area. The hilltop church formerly had a very large parish centuries ago and the current parliamentary constituency and the former borough council were named Islwyn after the church. The Reverend William Thomas (1832-1878), a Calvinistic Methodist minister from Ynys-ddu took his bardic name as Islwyn and is one of Wales' most famous poets. The village originally occupied the western slopes of the hill with the grove above it and the church and parish was originally dedicated to Tudur ap Hywel, as recorded in tydyr ap howel c1566, and also in Twyn Tudur, the mound that had been the location of a 12th century castle. The area was shown as Monythuscland in 1566 and as Mynyddisllwyn on the 1885 map of Glamorganshire by Eyre and Spottiswoode of London.