This is a very unusual image and clearly one that was used to identify police and military figures for future reference. A line of people stand in honour of a funeral or some form of remembrance. And while the photographer, perhaps unintentionally, captures their image it is later used to identify them. Who might have been privy to such information and why might it have been used?
Collection: Irish Political Figures Photographic Collection
NLI Ref: NPA PHOP1
You can also view this image, and many thousands of others, on the NLI’s catalogue at catalogue.nli.ie
Owner: National Library of Ireland on The Commons
Source: Flickr Commons
Looks like it might have been taken at Dublin Castle or at an army barracks in Dublin. It identifies a few detectives from 'G Division' of the Dublin Metropolitan Police and other military figures, so it was probably used by IRA Intelligence, perhaps under the direction of Michael Collins.
#4 Col Johnstone en.wikipedia.org/wiki/Walter_Edgeworth-Johnstone
The yard at Dublin Castle: Streetview.
Here is some info on Cas[s]maker, the Auxiliary.
National Library of Ireland on The Commons
NOTE: A number of comments, not really related to the image and not all reflective of the spirit/vision for this project, have been removed. Apologies if anyone's intervening comments have been errantly removed from the thread as a result