"Insula" literally means "island", and it's likely that the word itself arose from the Indo-European root "ins-" meaning "in" or "within".
This root likely reflects the idea of an island being a piece of land "within" a body of water.
So, the word "isle" has a long and interesting history, tracing back to the Latin word for "island" and ultimately to a root meaning "in" or "within".