Lookout or coastguard shelter at St John's Point, Donegal. I didn't venture up any further although it appears to be deteriorating badly - G7069 : Observation post gives a close up view and more information. The triangulation point - just a bolt not a pillar - is/was located on the roof of the structure but, as part of this has clearly caved it, it may be destroyed now.
A triangulation station, also known as a triangulation pillar, trigonometrical station, trigonometrical point, trig station, trig beacon or trig point. Erected by the Ordnance Survey.