Céad Míle Fáilte Romhat!

(100,000 Welcomes to You!)

You will be asked for tips: the gate admission gets you in, but throughout the day you will be asked by just about every performer to tip them for their performance. Of course tipping is totally voluntary, but the performers aren't shy about letting you know they make a living off their performances, and tips are an important part of that income.
Dress up and it's more fun. This doesn't mean you have to go out and purchase or rent a Renaissance costume. With a little Internet research and your imagination you can probably come up with an acceptable costume.
Expect bawdy humor. Many of the entertainers perform period entertainment such as singing, playing of medieval instruments, juggling, magic, etc. that is family friendly.
You are out among the elements. Most festivals are set in open meadows and forests. If it has rained recently, expect some standing water or mud. The toilets will probably be primitive. There will be no running water.
There are lots of ways to spend money. Again, part of the festival atmosphere is the many artisans, craftsmen and food merchants.