There is one at either end of the room over the doors.
A Fresnel lens is a type of composite compact lens originally developed by French physicist Augustin-Jean Fresnel for lighthouses. It has been called "the invention that saved a million ships."
The design allows the construction of lenses of large aperture and short focal length without the mass and volume of material that would be required by a lens of conventional design. A Fresnel lens can be made much thinner than a comparable conventional lens, in some cases taking the form of a flat sheet. A Fresnel lens can capture more oblique light from a light source, thus allowing the light from a lighthouse equipped with one to be visible over greater distances.
Most Irish lighthouses had Fresnel Lens which gave their lights a much brighter light over a longer distance than the LED lights.