Dublin Express Bus Question

We will be arriving to Dublin on Thursday, March 28th via car rental that we need to drop off at the airport. Given that we will be driving in from Dingle and hoping to add a stop or two for sights and lunch, our exact arrival time is a bit up in the air but most likely in the mid-afternoon range. One question is: for a family of four, how likely is it that we can drop off our car and then head straight to the Dublin Express bus location and get on the bus if our advanced purchased ticket is not the exact time (how full would the bus be in your experience)? The website indicates that we can do this as standby, but wanted to see how likely it is for the bus to be full: you can travel 12 hours either side of the scheduled time on your ticket on standby basis when traveling on our services between Dublin City and Dublin Airport. It is much cheaper to book online and especially much cheaper to book a round trip ticket, so somewhat motivated to do so. I am feeling optimistic but would appreciate hearing from those with experience.