Just came across this thread, I was looking into Patrick Cahill, optician. Here's my info on him: was born Patrick Keary 1853 to Patrick Keary and Mary Anne Butler of Ballybough Road. He was baptised 23 December 1853 in St. Agatha's Parish.
Patrick Keary could trace his ancestry back to Tomás Baintreabhachd Ó Ciardha of Fore (Co. Meath or Westmeath) who died in 1654. The pedigree is available online from O'Hart's work:
http://www.libraryireland.com/Pedigrees1/keary-heremon.phpO'Hart belived that the Kearys were meant to be a branch of the Cahill family of Co. Galway. However, Patrick Keary was also married to Elizabeth, daughter of Patrick Cahill of a well-known Kildare family.
I'm not sure why Patrick Keary changed his name to Cahill, but he died in 1928. He had sons and Cahill's optician on Wellington Quay was sold up in 1951. According to newspaper notices, he really did supply spectacles to several popes over the years.
Would love to hear more if any one has information.
Patrick Keary's son William Keary Cahill seems to have gone over to England where he married and died in the 1950s. There's an entry for William Keary Cahill in the index of wills and admins on ancestry.com, but i don't have access myself.