You have to do it in proportion to the size of the round.Buying a round every time you get a round will bankrupt you in no time when drinking with the Irish.
Basically if you get 3 free drinks you should also wind up buying 3. Now, that rule can be bent if you are actually done drinking and don't want anymore, but the guy buying wants the company. Plus I'm one of those super rich Americans everybody hears about so it would take the entire country to bankrupt me.