well if you're a Garda you could just park in the loading bay outside of Buswells like they all do. otherwise the multi-storey behind there is probably best.
it will take less than an hour to get to airport from there if you leave around 12. go down dawson street, turn right (there's no legal left turn) then follow road around to to the left down westland row, follow road around to left (Pearse Street) turn right at second set of traffic lights stay in second lane, straight across bridge then up gardiner street, turn right at dorset street then straight out to airport.