Echoing what others said, yes, very easily. I was there this weekend (Saturday) and we left much earlier than anticipated because we had done everything we wanted to. Probably depends if you want to ride the flats and some of the kiddy coasters.
We did The Quest once - didn't care to re-ride as it was your standard family boomerang (30 minute or so wait). Cú Chulainn we did once only too - just didn't enjoy it much at all (40 minute wait). Na Fianna Force 4 times (line varied from 5 to 15 minutes). The kiddy coasters we did not do as they had 60-minute lines, so if you do want them, I agree with others saying do them early. We're not a fan of spinny flats, and didn't want to sit on the bus to Dublin for an hour wet, so skipped Viking Voyage. We arrived at 10am and left at 3pm and felt like we'd been padding our time out.
There's quite a big zoo section which you can spend a couple of hours at, but even then I'd say it's very much a half-day park unless you want to ride absolutely everything multiple times over. They could do with better food and some bars (no alcohol is served on park), and another expansion or two like the Fianna Force/Quest one, really.