Grianan Ailigh, Derry
Grianan Ailigh (4000 years old) overlooks Lough Swilly and surroundings. You find it when you drive from Derry across the border into Donegal, direction Burt. Once you get to Burt look out for the Grianan Ailigh Visitor Centre on the left hand side. I do not really recommend to walk all the way up - it gets quite steep. Take your car, much easier.
I have been up there a few times when the sun went down, and that's pretty spectacular.