This walk is the long way to reach to top of Benbulben, but it is probably the one which requires the least climbing. You basically drive to Ardnaglass upper and park at the river. Here is a small parking space for a couple of cars.
From here, you follow the tarmac road which follows the small river until you reach a small bridge across the river. Cross the river and follow the bog road up to the end. This was the easy bit. Make your way through the bog towards the small river which comes down the mountain (see picture below, route highlighted in red) . You can actually see the trail along the slope. The first part is the steepest part and flattens once you reach the river. When you reach the top, turn right and follow the edge towards Benbulben summit.
Detailed trail Maps:
Length (km): Approx. 11 km
Ascend (m): 526m
Start point grid reference: IG -19757, 516813
Finish point grid reference: IG -19757, 516813
Ordnance survey map: OSI discovery series 25