He was not a civilian at the time, what I've found is that he was returning from West Africa where he had been with SOE or PWE in the region.He was returning to England to take up a position with the D-Day planning staffs.
There was nothign sinister about the aircraft landing in Ireland.It was a regular BOAC flight which went between Portugal and Foynes.G-AGES simply was very unlucky.That said the flight was of course not a strictly 'civilian' flight by any stretch fo the imagination, it carried among others a number of air force personnell who had been interned by the Portuguese but who had been released by them under the standard operating procedures between Portugal and the Allies.
I as hoping to find a photo of A F R Lumby somewhere to add to the information I got already.If you carry out a google and google books search you will find many small bits of information about him.
His military history is described in the book, The Deceivers: Allied Military Deception in the Second World War by Thaddeus Holt - 2004, Page 169:
In a footnote, Holt explains that Lumby was returning to the UK to take up a post with the ISSB having spent a year working for SOE in Africa, when he took over the seat of another officer and reporter, Malcolm Muggeridge at Lagos. His book, "Infernal Groves" tells how he learned that the one legged colonel who had swapped with him had been killed in a crash near Shannon. His service in Gold Coast is described in some detail by N E Lawlor in her book, Soldiers, Airmen, Spies and Update 26 June 2009 - Page 4 Whisperers. The correspondences to BOAC after the crash clearly state his employment by the Political Warfare Executive.