In the latter part of the Middle Ages (circa 1330 - 1403) Ynysforgan was the home of Hopcyn ap Tomas ab Einion. This clearly indicates the name Ynysforgan extends back further than the 15th century and given the 'modernisation' of language it's conceivable its name is derived from old Welsh.
During the latter Middle Ages (circa 1330 - 1403), Hopcyn ap Tomas ab Einion resided in Ynysforgan. This suggests that the name Ynysforgan has a history older than the 15th century, and it is plausible that its origin can be traced back to Old Welsh.