The Frank (born Francis) Finan noted as grandson in the 1911, was Maggie's child. Given Maggie was in the 1901 census, it would seem odd he was missing from it as well as Celia/Cecilia. I wonder if either/both were in hospital, although so far, no obvious C F (that is, initials only), appear in the 1901 for Sligo that would match your Cecilia/Celia by age. Also, where was Maggie by 1911 (not seeing a death/marriage), or is she the 'Mary' listed?