It's a huge stretch to say an 8% increase in August can be taken as a proxy for the whole year. You could have had a new route starting this August and one falling away last August 08 which would inflate that number.
Regarding Belfast and Ireland, to avoid unproductive and pointless arguing, let's rephrase your (provocative) statement to "Belfast City to be 3rd biggest (by passenger numbers) on the island of Ireland".