Mick

The most plausible derivation I can find is in Dolan's Dictionary of Hiberno-English where he says (with minor technical material removed):

Reek : a heap, stack (of hay, turf etc.) from dialect English and Middle English hreac, , rick.

Application to a mountain is, therefore, by extension - obvious enough with Croagh Patrick, given its shape. Less so for McGillicuddy's!.

Mind you, my own experience is of only hearing the short "i" rick form in regard to turf.

Regards