Get the bus from Bus Aras (Main bus depot) on Amien Street (across from train station)
Then get the bus from Drogheda to Newgrange/Donore. Runs fairly frequently and leaves you right to the carpark at Bru na Boinne.
Be very aware though that the last tour of the day arrives back too late for the last bus in typical Irish fashion, means you can meet some locals while hitch-hiking home. I've often given people lifts back to Drogheda at closing time!