Check that you can take your hire car into Northern Ireland from Dublin.
Are you arriving jet lagged into Dublin - if so give yourself a day there to recover rather than jumping on a car to head to Giant's Causeway. G3rryCee's itinerary would work well.
Giant's Causeway is best seen at low tide when more of the stones are uncovered. If time allows, it is also doing the walk up behind the Visitor Centre to see from above, when the tourists look like ants scurrying around below... The ruined Dunluce Castle is also worth a stop as you drive past.
Agree, see the Cliffs from above (be warned it is a hairy drive up them) rather than from the water. (They aren't as dramatic from the water as the Cliffs of Moher. )