Visibility is an issue, whether they could see the Blackrock Light definitely is an issue.
As I have tried to explain, a letdown of this sort using the SAR modes to 200' without the aid of NVGs, is an instrument procedure. How often do you look out of the window when you are flying IMC?
The lighthouse may or may not have shone in their direction during the crucial period before the crash but they weren't expecting it to be there and therefore were not looking for it. Instead, they were doing as trained and monitoring the MFDs, albeit with the possibility of a radar contact obscured by WP data.
They may also have had their landing lamps on which would produce many reflections but all they were expecting outside is sea, sea and more sea since they were a long way from the coast and Blacksod.
The lighthouse beam is irrelevant in all aspects other than it might have been the last thing to save them - if they had been looking for it.
The fact is the rock became visible to the FLIR operator in time to avoid it but that didn't happen due to procedural focus by the flight crew.
Concentrate on why that happened rather than obsessing about the lighthouse visibility.