Hi Mrs V,
Im not sure how you plan to get there (car, bus, taxi).
But if you are taking a bus you need to take a bus from heuston to city centre - maybe 15 minutes to catch bus and get there. Then either take any of the 15 buses (except the 15a) from the side of Trinity College (across from D'olier St) and the journey is around 30 minutes (depending on traffic), or a 49 or 65 bus from the quays (opposite SIPTU) - again, roughly 30 mins journey.
A taxi from Heuston would be quickest - 30 mins total, with a cost of maybe 2oish euro or thereabouts?