Buy a ticket to watch a game at Donnybrook Stadium. If there's nothing scheduled while you're in town, you might like to check if there's a tour of the facilities.
You'll be spoiled for sightseeing choice when in Donnybrook. Energia Park is a good place to kick off here, but there are loads of other worthwhile sights to consider. Guinness Storehouse, Trinity College, and Aviva Stadium are near the sporting facility and and will cap off your day nicely.