Póg - kiss
mo - my (causing lenition of following consonant)
tóin - arse.

Mac Mahon or Mahoney from Ó/Mac Mathghamhna (Mod. Mathúna). Mathghamhain was an old word for bear (another was milchobur 'honey-lover'). Gamhain is still used for a one-year old calf.

Connacht Irish stresses the first syllable of words; Munster Irish stresses the second syllable; therefore, when Munster speakers say "Mac Mathúna" it is phonetically equivalent to "mac mo thóna" in Connacht Irish (ó becomes ú when close to a nasal 'n').

Therein the joke of the butt.