The '-igh' ending of 2nd conjugation verbs is pronounced /ə/ before a personal pronoun, i.e. as if "Ar chuimhnigh sibh" were written "Ar chuimhne sibh". This is true for all dialects.
For some reason I thought I had learned that 'ar' eclipses the same way 'an' does, and that 'ni' and 'nior' lenited. Am I wrong about this?
An and nach eclipse, but ar and nár lenite (except in the past autonomous, which gets no mutation — excepting some irregular verbs’ past autonomous forms, which do lenite).