I managed to track down a phone number for them (also not on the website!) and rang. The opening hours are 11am to 9pm on weekdays, to 8pm on Saturdays and to 7pm on Sundays.
The website is similarly vague about buses, it only gives the buses that go to Ballymun, not particularly to Ikea. I emailed Dublin Bus (because the information is not on their website either) and they said the 13 and 13A stop outside Ikea, and when the road is finished then the 140 will as well (but it currently doesn't).
Have to say, the Ikea Dublin website isn't exactly inspiring confidence - surely this kind of information should be easily available.