Okay, this one is pure conjecture on my part. An 1877 official Admiralty booklet by Richard Hoskyn RN, entitled Sailing Directions for the Coast of Ireland, shows that the River Foyle was well lit with lighthouses, pile lights and a fishing smack. Over time, most of these have been replaced. Moville is the only pile light left in Ulster, with an abundance of pole lights (as the single structure above) lining the two banks of the Foyle. The light above is called Whitecastle after the townland of that name on the western shore, just north of Quigley's Point. But what of the eight poles sticking up out of the water next to it. Does that not look like the remains of a pile light?
The above publication lists a pile light at Whitecastle - "Whitecastle Lighthouse, on piles and painted black,lies on the south-east side of the channel off White Castle and exhibits a fixed white light at an elevation of 26 feet." Another 1870 publication, says this light was erected in 1848.
Of course, there may be a better explanation and I can't quite figure out why they would half-demolish a pile light but for now, I rest my case, m'lud.