found an interesting entry from Thom's of 1914 ...
Roe McMahon, artist and photographer, 11 Harcourt street, 37 Grafton street,
and 25 Sackville at, upper
- res. I Terenure park, Terenure
is this father of the Henry Roe McMahon that gormangenealogy found ?
55 Sackville St (aka O'Connell St) listings for both 1894 and 1914 do mention a photogtrapher but does not seem to be connected to the McMahons at that time.
55 lwr Sackville st,
Chancellor and Son, watchmakers, turret & house clock and carillon
manufacturers, jewllers & nautical instrument makers.
John Chancellor
optician and photographer to the Prince of Wales
res. Craiglands, Dalkey
Shane