Actors are (usually) expected to perform all 8 shows. Like any regular job, they have a certain number of sick days, personal days and of course, they can get permission to take additional time away from the show, without pay. There are some roles in some shows that contractually the actors only perform 6 times with an official ALTERNATE. This is not the case with Once.
Other than that, they are there without any pattern.
If we're not having fun, then why are we doing it?
These are DISCUSSION boards, not mutual admiration boards. Discussion only occurs when we are willing to hear what others are thinking, regardless of whether it is alignment to our own thoughts.