Bríd Mhór

Depends on the sound in question. English ⟨d⟩ and ⟨t⟩ are rarely eclipsed and never lenited (in Connemara at least, and I would think farther north likewise; I’m not sure about Munster, where their slender ⟨t⟩ is closer to an English ⟨t⟩). Things like “i mBoston,” “muintir Mhassachusetts,” “i gCalifornia,” “i bhPhiladelphia,” “i bhFlorida,” &c., are common. The closer a name is to following the rules of Irish phonology, the more likely it is to be treated as Irish.

That is what I’d say too. I would also write them that way. Although it is probably against the standard rules.

In Conamara it’s common to lenite Sara too. ” A Shara”. And ofcourse, “a Mhichael”, “a Mhary”, but not “Tom”.