I think DannyM8
might have finally cracked this one. He proposes John A. Sinnott as the older man and his nephew John Nicholas Scallan a recently qualified a solicitor as the younger man. Sharon thinks the photo may have been taken at the time Scallan joined his uncles firm.
John N. Scallon gets married in 1909 and by that time the marriage notice mentions his late uncle - John A. Sinnott.
I note that John A. Sinnotts father was Nicholas.
Sharon finds John Nicholas in the 1901 census as a Solicitors Apprentice.
Therefore I am going to use the People Identified
tag, which has been redundant for a while!!
Niall McAuley and sharon.corbet
Think the younger man might be Michael O'Dempsey and they point to a document that shows he was shot as a spy in 1922 - see here, it is worth a read
Having made the error of posting this photo for a second time and then seeing the interest it generated we may do the same again - watch this space!!
Photographer: A. H. Poole
Collection: Poole Photographic Studio
Date: Circa 29 September 1906
NLI Ref: POOLEWP 1616
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