I've used them twice recently and they've been around 40% full both times. The passengers seem to be evenly split between airport workers and passengers. The early morning service may be aiming as much at employees who start an early shift at the airport as aiming at passengers. I know the Bus Eireann bus carries as many passengers to the Airport Business Park as it does to the airport, but Skylink doesn't stop there so it's missing that market.
One problem I find with the Skylink service is that because they operate two routes, it's hard to know when and where to get picked up in the city centre without checking their website. A slight route tweak could send both routes past the bus stop on Anglesey St and guarantee at least one spot that gets a regular service.