One of my favourite Welsh names, hovering at the top of my list for future daughters. Criminally underused in my opinion and this is coming from a Welsh person from Wales; I've only met one Esyllt! I love Essie for a more "conventional" nickname.
Esyllt in OLD Welsh means 'fair and clear' or 'fair to look at' from syllt - fair to behold, from syll - sight, and 'e' which is possibly a pronoun, but is male, meaning 'he is fair to behold'.