Half the reason placenames across wales were spelled incorrectly in the past is due to English administrators and illiteracy.
I've seen Dollgelley in old Parish Registers and Censi for example.
I've heard Dol-ga-loo from tourists. I've also heard Aber-wrist-watch and Penrhyn-cock amongst others. Its sad how many people think it is funny to be completely ignorant of these things yet would take offence or be the first to correct if the shoe was on the other foot.
Just top put the record straight and get back on topic the answer is:
Dol-gell-eye, but the locals will often just use Dol, Mach or Aber for example. 'Llanfair PG' possibly being the worlds best shortening of a placename Lazy bastards, us Welsh.