Wouldn't this be ochtó bliana since bliain has a special plural form used for counting? Or am I forgetting part of the rules?
Ochtó bliain is correct. A summary of the rules for nouns such as bliain can be found here.
Go raibh maith agat.
"Fiche bliain ag fás" - I should have remembered this one.
Does this mean, "He is eighty"? Or am I missing something?
This is literally what they say in Irish (also in Spanish and lots of other European languages), but in English??