My Irish mother pronounces
Mavourneen as "mah-VOOR-neen." She's from western County Cork, in the extreme South. The area has its own distinctive accent, so I'm not sure if this pronunciation carries throughout
Ireland. But it makes perfect sense to me given the spelling.
Montserrat is Spanish. I would say it as "mohn-sehr-AHT," but I'm not an expert. I do know that the first "t" is silent.