The monument known as Grianan of Aileach is a great spot when it's clear. When the gate is closed at the bottom it's still ok to walk up. Light pollution is indeed quite intense from Derry and the surrounding villages, but on a very clear night, the pollution can be quite well controlled in Adobe Camera Raw.
Here's a photo I've attached from August 2013 when some of us from Buncrana Camera Club went up. Justin MacLochlainn did the torchwork, it was a beautiful night! Nikon D600, Samyang 14mm, f3.2, iso800, 20 second exposure.
Inishowen is indeed a great spot on a clear night. Lisfannon and Luddan Beaches in Buncrana are good locations, but the further out you go the better, Dunree Fort is fabulous, The Gap of Mamore / Mamore Gap is amazingly dark and Leenan and Urris are wonderful to.
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