It's the job of the teacher to ensure that beginners are always paired up with a more experienced dancer. Beginners who come as a couple will be split so they can be helped by those who are used to the movements and steps of the dancers.
Don't know a dance?
If you do not know a dance do not go into top position, unless you are with a partner who does. The top couples lead the dance, and if they don't know the dance, they could spoil the fun of all. This is of course less important at the teaching sessions, but at a ceili, you need to think of the others. At a ceili, if you don't know a dance, just tell your partner and the others in the set that you are unsure, and help will be willingly give. No one knows all the sets, and everyone needs help at some point.
Dancing is a great occasion to meet not only old friends, but also to find new friends. However… when the teacher is explaining the set, do not talk. The others in the set will not be able to concentrate, and will have great problems dancing when the music starts. Remember, Irish set dancing is team work, so think of the others.
If you are sitting a set out, please talk very quietly. You know yourself what it's like when someone talks loudly when you are trying to listen to the caller, so think of the others.
If there is not enough place for some dancers, then the dancers on the floor will be asked to swap with someone sitting. That will be done for every figure. Figures that are difficult or much liked will be danced two times to give everyone the experience and fun of dancing them. Don't let the teacher ask 3 times for dancers to swap out with those sitting. Everyone quickly sees those who hog the dance floor and refuse to change places with those who are sitting.
This is all really just common sense and being polite.