Top-cover for deep-water ICG ops is normally provided by an Air Corps CASA, for example:
IAC CASA top-cover
I don't know why that wasn't the case last night unless it wasn't just top-cover but also SAR assistance. Edit: apparently this is a touchy subject, fixed-wing top-cover is less frequent than desired. When one heli accordingly has to support another it is technically Mutual Assistance in CHC / ICG lingo.
What particularly puzzles me is why the Dublin heli 116 was dispatched across the entire width of Ireland, thereby needing to refuel before proceeding on station, instead of the Sligo unit 118 which could have been on station far faster and then be back-filled by Dublin.