It is about 3 miles of walking to Cosmos. Easiest way from Shamrock would be a route taxi going north and get dropped off at Cosmos. Walk through JUTA's parking area, Cosmos, then turn NORTH (right). Start walking until you like what you see.

Another option is to have the driver go a bit further and drop you off before hitting Beaches Sandy Bay at the actual entrance of the park.

If you like walking on the beach (like I do), then you'll enjoy that walk. You can also hire a driver for the day to take you to really isolated beaches along the south shore.