Looking at this again, I missed the fact that the lighthouse is outside of the coastline which is unlikely and going by pics it is of course above the MHWS.
https://www.geograph.org.uk/near/55.73032%2C+-5.0045
If you compare it to old maps overlayed with OSM or Bing, it looks like they were correct a while ago but modern OS has gone wrong.
https://maps.nls.uk/geo/explore/#zoom=17.2&lat=55.73115&lon=-5.00616&layers=193&b=8