I have also applied for a position as an ATSOA at Stornoway airport.
If offered an interview with HIAL,take it! See what they have to say, remember it's your interview as well. Ask about career prospects in H&I. An ATSOA at a HIAL location would require you to become a FISO for Air Ambulance flights outside published ATC hours. There is also a requirement to hold a MET Obs certificate.
A small number of HIAL ATSAs / ATSOAs have in fact been offered ATCO training and have passed out successfully. That said the numbers are small, as the ATSUs are small.