It depends. For example in Letterkenny, there are 3 buses parked for route 32 in the morning usually. 2 SP's and SE 13 usually. The driver on the 05:30 is allowed to choose between the two SP's, but is not allowed to take SE 13. For the 07:20, if SE 13 is there, you must take it, unless told otherwise. This just leaves one bus then for the 09:20.

So it depends on what run you are on whether you have a choice or not.

There are also two buses parked in Letterkenny each morning for route 64. The type of bus differs from day to day. You may take what you please on the first service there.

It is up to certain garages about the allocations of buses within their garages/depots. Sometimes, even if you are first, you may not have a choice. An inspector may request you to leave or take a particular bus. It differs daily.

That's just me speaking about this end of the country anyway. I don't know how it works in other places, but I imagine it would be a similar set-up.