I am staying in Dun Laoghaire and need to pick up my rental car when I am leaving that area. I pick it up on Haddington Steet in Dublin. Does anyone know which is the closest DART stop in Dublin to Haddington Street?
Probably Grand Canal Dock, although you'll still have to walk a bit.
DART functions more to transport people into the center of Dublin from the outskirts, rather than a comprehensive transportation system within the city (like the Paris Metro, or London Underground).
If you don't want to hoof it from the DART station (distances in Dublin aren't all that far) you could check out www.dublinbus.ie there are a couple routes that should get you there, or at least close.