Cuil is the Gaelic word for both knowledge and hazel
On the main page they say "old Irish" instead of "Gaelic". So does cuil mean either "knowledge" or "hazel" in any language? The Early Irish Glossaries Project has cuil: "fly; flea, gnat". "Hazel" is coll (cognate with hazel), altho it was sometimes spelled cuil, probably in inflected forms. MacBain's has Old Irish cuil: "corner, recess" (also). Here's focail.ie on cuil: "fly, bug".
The Cuil site also implies that the word, whatever it means, is connected to the name of the legendary hero Finn McCool, which they spell McCuil. This looks like a folk etymology; the name is normally spelled Fionn mac Cumhail in Irish Gaelic.
Perhaps they should have googled it.