Before the Irish CG got the 92s, any long range jobs were given to the RAF at Valley or Chivenor because we had the better range,
There were only 2 refuel sites on the West coast - Castletown Bere and BlackSOD - I can think of no reason to add another on top of a 300' rock.
We used to organise periodic recces for our crews of the most likely refuel sites, Blacksod for Valley and Castletown for Chivenor so I would think that the Irish crew would at least have had a working knowledge of the sites, it's not like any of them were newbies.
I don't know the CHC SOPs so they may have had a preplanned letdown and a home-grown plate for an IMC approach to the Blacksod area - otherwise it would be similar to how I described it earlier with a combination of radar clearances and the SAR modes of the autopilot to transition down safely beneath the weather.
It is possible they got down on the Westerly track and then turned downwind to transit to the refuel site at low level but I still can't believe they flew into the rock without realising it was there.