The Coumshingaun Lough car park is the best place to start your hike and you can simply type the name into Google Maps to get the exact location: the car park is not big and at weekends it can get packed not only with people hiking up to the top but also with people going to enjoy the lake from the bottom. It's advisable to arrive on location quite early in the morning but do not worry: if you can't find a parking space in the cart park, there are plenty of options in the proximities and you won't spend hours looking for a place where to park.