Ah, fascinating. So, the original "foil" might have been one that in Irish was pronounced, say "fweel," or "fee-ahl" or something else, each of which might have been different words with different meanings.
We have examples of that even here in the States, where "Gnaw Bone supposedly came from a mishearing of the original French name of Narbonne. Similar mishearings have left Americans with Low Freight (L'Eau Froid), Lemon Fair (Les Monts Verts) and Bob Ruly (Bois Brulé). Native American names were more often than not mangled into forms more easy on the European tongue: Oxopaugsguaug became Oxyboxy; Moskitu-auke to Mosquito Hawk; and Waakaack to Waycake Creek." Well, supposedly.