[Facts] Re: What does the name Yenthl mean
I agree with Phyllis, I don't believe this is a Celtic name. Now that you've at least told us it's a feminine name, I would say that it may be a form of the Yiddish name "Yentl" which you already found on this website.
The name "Yenthel" appears on a number of Netherlands websites, although unfortunately, they don't give a meaning for this name.
It's too bad your girlfriend can't remember where she'd heard this name that she gave to her kitty. Maybe if she thinks a little harder, it'll come to her eventually.
-- Nanaea