Howdy ...
If you look at a map of Marlay Pk, the concert entrance is from Grange Road.
I reckon you have 2 options;
1. Stay in the city & get a bus out to Marlay Park (
www.dublinbus.ie )
2. Grab Accomodation near Marlay Park, leave the car there.
1. Get the 48A from O'Connell St, to Broadford Rd, then walk up Ballinteer Avenue, reverse for getting home .... This would be suspect, as there is no mention of Concert Buses on the Dublin Bus website, so you may have to leave the concert early to catch the last bus to town ...
2. Try this crowd (
http://imiresidence.com/marlay_park.php) .... Getting to & from the concert would be easier than to/from the city centre esp. as you are unfamiliar with the area so no messing around with buses.
I presume you are coming from the North, when you pass the Airport keep an eye out for the M50 motorway (southbound) & follow it all the way around to exit 13 (There is a barrierless toll on the M50, so just pay in a newsagent before 8am the next day)