The equivalent of "A' Bhreatann Bheag" in Irish/Gaeilge is "An Bhreatain Bheag" and it means Wales! I find that so interesting how they have different meanings
I find it interesting that Britain has no article, but when it has an adjective, it does - Breatann BUT a' Bhreatann Bheag, a' Breatann Mhòr. Any reason? Or just accept that it is so?
Probably just accept it. You'll find later other inconsistencies like this, eg that while Alba has no article in the nominative, it has one in the genitive, so it's 'Alba/Scotland' but na 'h-Alba/the Scotland's'.