I have been waiting for Llanfair to lleap in here, and explain to us the complexities of the Welsh language. Consonants undergo strange mutations so that foel as in Foel Fenlli, becomes moel as in Moel Siabod,but both mean "bare, or bald hill". Mynydd means mountain, but there is nevertheless a subsidiary summit of Cader Idris called Mynydd Moel! Yr Wyddfa is of course Snowdon,meaning "the mound or tumulus", and BTW you should never, ever refer to Mount Snowdon.
Speaking from a lifetime's experience of walking the Welsh hills and mountains, I can only add that Welsh placenames can be a minefield for the unwary, and I have more than once climbed the wrong mountain due to topographical confusion, or as my little sister pointed out "You're lost, aren't you."