That just got kind of dark.
Just what I was thinking...lol
Well! That's rude.
Why is "ar' here?
Because bagair ar is a phrasal verb meaning to threathen. You can hear an echo of it in the phrase "to threaten the law on someone".
why has máthair mutated with a séimhiú rather than a urú here?
1) The singular possessive adjectives mo and do cause a séimhiú, not an urú.
2) Words that start with m don't take an urú - ár máthair", leis an madra, i mála beag*, etc.