There's a slip road to the right for the LUAS, watch for the signs after Bewleys Hotel.
That slip road is no longer there. Stay left and take the turn for monastery road, and take the new bridge over the N7 into the Luas car park.
after a long search I got this map of the new access to luas car park
http://www.m50.ie/index.php?page=29-11-08-layout