For short strikes, people tend to manage around them, e.g. by working from home, shopping on a different day, travelling the day before, etc. On such days, overall travel demand is usually down quite a bit.
It is longer strikes that become problematic on a grander scale.
The NTA journey planner will usually adjust schedules to account for days where operations are abnormal.
There is a map here of operators that serve Dublin Airport from around the country. Most of these will also serve Dublin City: http://www.transportforireland.ie/wp...al-Routes1.pdf
Looking at it, it looks like the worst affected people will be those using city / town and commuter services or using cross-radial Expressway services.