Blackrock is only 12m West of Blacksod, is a potential landmark (lighthouse)/waypoint, hazard (300' cliff) and has a helipad; so is likely to have featured in some way in the approach - but I, like most of us here, have no facts to offer. The crew on the other hand were both highly experienced and presumably knew both these locations/approaches well from many years of practical flying. According to this report they had been there six days earlier
http://www.irishexaminer.com/ireland...er-445271.html
I don't think there is enough information yet to support any theories as to why this happened - there are just too many possibilities and hardly any evidence. It will become clearer in the next days and weeks but, for now this tragedy is simply unexplained.