It’s the Poolbeg lighthouse. You can google images. It’s built almost level with the sea, not on a high rocky cliff. This picture was obviously taken during a storm at high tide. They are desperate to deceive.
A lot more details here including how the sea wall dates back to 1795.
http://dublinportblog.com/2013/10/the-great-south-wall-poolbeg-lighthouse/
I found this bit interesting, and of course, no-one seems to use that reference mark now...
Historically, the Ordnance Survey Ireland used the low water mark of the spring tide on the 8th April 1837 at the Poolbeg Lighthouse as a standard height for all its maps, a practice which continued up until 1958.