Barnesmore Gap (, meaning the big gap) is a mountain pass or gap (elevation 117 m) situated in the Bluestack Mountains, County Donegal, Ireland. The main Donegal to Ballybofey road, the N15, and former County Donegal Railway run through Barnesmore gap, acting as the main route between south and north Donegal. It is an area of complex geology, but its main feature is granite formed in the Devonian period, 400million ears ago. The gap held glaciers in the Last Glacial Period flowing to the Atlantic through what is now Donegal Bay, up to about 13000 years ago.