there's a marriage of a George Penston listed in the Civil Index :
Name: George Penston
Registration district: Dublin South
Record type: Marriage
Registration date - quarter and year: 1876
Volume: 17 / Page: 764
He could possibly be Fanny's father... the district is correct for a person living in Thomas st. There are a few children with father George Penston listed on IGI - maybe from George's 1st marriage :
Julia - 2 Nov 1864 , Dublin
Sarah - 31 Mar 1866 (0786), Donnybrook, Dublin
Mary - 9 Nov 1867 , Dublin
John Joseph - 6 May 1871 , Dublin
parents George Penston & Catherine Byrne
amazing aghadowey .... we independently came up with almost the same response! sorry for double posting
Shane