You have now set me wondering!
Given that the title of the postcard refers to the National Volunteers, it seems that FX Martin's caption in his book 'The Howth Gun-Running 1914' - " "Volunteer officers receive orders before the march.................." may not be correct - since the 'split' in the 'Irish Volunteers had not yet taken place. I understand the split occurred shortly after John Redmond's 'Woodenbridge speech' on 20 September 1914 and therefore the name National Volunteers (INV) had not come into existence.
Alternatively, is FX Martin's caption correct and the INV 'hijacked' the photograph and had it made into postcards after their formation? The only thing I know for certain is that the postcard is postmarked 31 October 1914 or could that have been manipulated as well?
I do like your idea about photoshop - looking at the photo you have posted looks to me more as if Monteith was 'lifted' from the Volunteers photo and 'planted' in yours rather than the other way around. Monteith's legs and feet look to be in focus with the surrounding ground in the 'group' photo whereas the ground on which he is standing in your photo is not. What do you think?
The photograph that I posted was taken from a slightly different angle from the one in FX Martin's book, Monteith is partially obscured by Ch Instructor Bodkin, but the composition and subjects are identical even Monteith's 'pose'!
I also notice that Monteith's cap badge is different from that of the other officers in both group photos, his looks like an Irish Harp. A member of a military history list suggests that the cap badge is that of Dublin Regiment of the National Volunteers - you can see an expample on 'Memorabilia from The 1916 Easter Rising, its Prelude and Aftermath' at .......
http://www.theeasterrising.eu/120Volunteers/NV.htmthere is also a copy of the photo I posted as well.
(My grandfather is still looking at the camera in the FX Martin's photo & yes, he did join the RDF in 1915.)