The so called "Cairo Gang" photo comes in fact in two varieties, one with the Getty Image Archives (top print) and is credited to a Sean Sexton, and another attributed to the Piaris Beaslai collection at NLI (bottom print). Beaslai was a cousin of Lily Merin (or Mernin), one of Michael Collins' moles in Dublin Castle, who passed much useful information to Collins. He was also, worringly from the point of view of spin, IRA Director of Publicity
It could have come from Reynolds who was supplying Collins with information for money. Reynolds may or may not have been a double agent. Had he been a double agent, the photo could have been staged for him to pass on to tyhe IRA to help his credentials with them.
They are two different prints from the same negative, but note different lettering of the numbers, even though the numbers are the same.
It may have been taken in the narrow alley in centre bottom of the above overhead shot
And this alley close up
The collection of material held in Dublin, Military Archives (The Bureau of Military History) and contained in a scrapbook compiled by I.R.A.intelligence,1919-21,including intelligence in Collins papers on, special gang " F"coy aux div, giving names to the numbers in the in the photo.