Albion is an ancient Greek term for the island of Great Britain - it is also later translated into Alba, the Scots Gaelic name for Scotland.MattyG said:Albion would be Eire, Wales and Scotland, plus locations that don't get a province, like the Isle of Man.
Albion shouldn't include IRE, simply Great Britain (ENG, WLS, SCO)
Britannia is more or less the same thing, owing to the ancient Roman terminology for Great Britain - Britannia Superior being the south of England, Britannia Inferior being the north. The capitals were London and York respectively.
Scotland doesn't really come into it - the Romans never conquered or administered it, and Hadrian's wall was built for a reason.
As far as I'm concerned, Ireland is a totally different issue and unless you're going with the traditional Great Britain event chain (which, your mod being as it is, I would presume not) you shouldn't have any one country tag having cores on the entire British Isles.
If you want my opinion, I suggest you go with Britannia, and give it cores on Great Britain - but not Ireland.