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.
In the latter part of the Middle Ages (circa 1330 - 1403) Ynysforgan was the residence of Hopcyn ap Tomas ab Einion. This suggests that the name Ynysforgan predates the 15th century and, considering language evolution, it is plausible that its origin traces back to Old Welsh.