So maybe it is dependent on the suffix... is the suffix "-llyn" a variation (or origination) of the word "land"? In that case, we might simply call them "Epons".
Actually 'llyn' is Welsh for lake (so I hope you are all rolling those lls properly)
But given how much of a pain that ll is to say I kinda like Eponi but as in Epon-i not as in E-pony :p
I also like Eponlli (with the ll)
But there is, it turns out, a whole Wikipedia article about alternative names that the rest of the world calls us Brits so I guess it is up to the rest of the continent to find something that they can call us
