Know the rules before you place a bet with any bookmaker (or anybody else for that matter). If a game goes to extra time because of a draw at the final whistle, the match is usually declared a Draw. Whatever happens in extra time does not count on the bet made.
Friendly Betting (gambling) even amongst friends can break friendships. Never assume anything when you place even a friendly bet. The wording of even a verbal bet can lead to disputes.
This is a carry through from soccer betting. The possibility of a draw is huge in soccer and betting is dominated by the "90* mins play" market, which allows seemingly generous odds to be available for the "win". So almost by force of habit Gaelic betting is primarily the 70* mins market. But the match market is also available at slightly worse odds.
* it is described as 90/70 mins but of course includes injury time but not extra time