From the Port Road, Derry
A lot of poring over Google maps beforehand convinced me that Ballynagard Lighthouse and other lights at Otterbank and Brook Hall were best viewed from the east side of the River Foyle. The light was originally exhibited through a window near the top of the structure but that seems to be blocked off now, replaced by a solar powered light on top of it.
Focal plane 6 m (20 ft); white flash every 3 s. 7 m (23 ft) concrete tower Lighthouse painted white with a green base. It has a range of 3 nautical miles
From Otterbank Road, Derry
From Culmore Point Lighthouse downriver