Is this a lighthouse? My Ireland road atlas says it is, but I can find no reference elsewhere to this light in Strangford Lough. This view from the harbour area of Killyleagh.
This is apparently Town Rock. A sector light was installed on top of the tower in 2011 as a means to augment the poor navigation aids in the Lough.
" Built sometime in the 19th Century to help guide vessels into Killyleagh, it would originally have had an open fire burning atop at night or in bad weather rather than a light."