The reference to King John was introduced by John Ferrar in the late 18th century. It was always known as Limerick Castle or sometimes King’s castle more locally.
Ferrar was the grandson of a Williamite settler so it’s interesting to see where you take your cues from.
But I’m glad to see that we should dismiss what history departments teach (not that they did) and we should trust whst the twitter or insta picture tells us 