Mitchaa
I'm really struggling to think why a standard approach to Blacksod would be via Blackrock, as you say you think happened. If it was good VFR you'd not need the Blackrock light as you'd have Blacksod. And if you were IFR why choose a rock to pass over that was 300ft higher than anything else within several miles? It's not as if it had a VOR or NDB on it. Seems a bit of a stretch to think it was perhaps because you could identify it on radar better than anywhere closer. And then surely backed up GPS would be more reliable anyway? Ideas - anyone?