I understand that the word exists in Welsh, but that it's a modern loanword from Gaelic referring to Scotland.
Albion first appears in ancient Greek accounts along with Ierne as being part of the British Isles. The Romans don't appear to have used the term much, preferring Britannia though the term survived enough for Aethelstan in the 10th century to use it as part of his title. Presumably the word survived in Gaelic or maybe Pictish which is where the modern word comes from, but does anyone know if the word has any historic use in Welsh?
I can hazard a guess that Roman rule replaced the native word in common use, much like Welsh adopted the use of Welsh to refer to themselves instead of the native Cymry, though it's only a guess.