Ballynagard light, on the west bank of the River Foyle
The river flows through Derry, where it widens out, before narrowing again and snaking up to Culmore. Just past Culmore, the river turns into an estuary and on the west bank you enter the Republic from the North at Muff, which gives me the opportunity to show this wonderful photo of the highly-fortified customs post at Muff in the early days of our nation.
The accepted colouring scheme at the time was red for lights and buoys on the starboard side coming into port and black for the port-side. A 1917 nautical guide says that Ballynagard was white, with a red base, so presumably it was the same as the present light in the top picture, but with a red base. (The colour scheme in Ireland changed from black and red to red and green in the 1940s)