If you were driving from North Sydney to Manly you'd be taking Military Rd until Spit Junction in the suburb of Mosman, then turning into Spit Rd which brings you to The Spit and The Spit Bridge.
The Spit Bridge is an opening bridge and is closed to road traffic a number of times daily when the bridge opens to allow ships to pass unhindered through that section of Sydney's Middle Harbour.
Almost directly east of The Spit is the suburb of Clontarf where you'll find long and narrow Clontarf Beach with playground and barbecue facilities, a pool, and a marina.
Driving to Clontarf Beach from The Spit is a little tricky as you turn west to get east. Cross The Spit Bridge north and up the hill, turning left (west) where it says Clontarf/Frenchs Forest.
At the roundabout do a sharp left (east) into Ethel St, then right into Kanangra Cres. Turn left into Perrone Rd, then sharp right into Sandy Bay Rd and — how about that? — you're there.