A number of Easter Rising books/documents touch on a British Army Cadet G F Mackay as being captured at the start of the Easter Rising and held at Boland' Mill until the end of the Rising :
Max Caulfield's The Easter Rebellion
There is an actors voiceover providing a recording of Cadet Mackays' experiences (based on an account in the Ampleforth Journal of 1916)
In an educational resource pack, he appears to have written to Eamon De Valera congratulating his former captor on becoming President in Ireland; he appears to have become a Lt Colonel
He stated he was 18 in 1916.
The nearest name that matches is George F(rederick) Mackay in the 1901 census for Ireland
This chap and his brother Charles (Joseph) MacKay are at Ampleforth school in the 1911 census and are then referenced in the Ampleforth Journal re war service, both in the Leinster Regt and then RFC (G F Mackay is also referenced as RAF). In the 1911 census, George is listed as born Co Cork and Charles born in Co Westmeath.
A digitised record for George Frederick MacKay from the UK National Archives gives his date of birth as 7th June 1897, Mitchelstown, Cork/Limerick. Shows addresses in Hove, Sussex and Accra, Gold Coast, West Africa.
His RFC record has not been digitised yet.
There is a WW2 war grave that mentions a Lt Col G F Mackay living in Jersey.
and this appears to be George Frederick MacKay.
George Frederick MacKay died in Brighton, England in 1968 aged 71. His wife Sonia Mary MacKay died in 1978 aged 76.