The only thing I've noticed that visitors to Ireland of partying age get wrong, consistently, is the 'rounds' system in a Pub.
If you are in a pub and drinking with anyone, stranger, friend, classmate, workmate it doesn't matter and you are about to order a drink, it is extremely rude not to offer to buy your companions a drink too. This is not some trick, you'll notice immediately that people will offer to get you a drink when you are out with them. They are not being super nice and friendly to a visitor, they are doing it under the implicit agreement that you will buy the next drink for them.
If the group is too big it will generally break up into smaller rounds, generally appropriately sized to the amount of drinks the group is expecting to have that night. If there are 10 people out it will likely break up into approximately 2 to 3 rounds, based on who arrived when, and who has finished their drink faster. When you find yourself in a round, stick with it. Don't start buying drinks for someone in an