Then, assume that Dennis Sullivan was born in Rotherhithe, 28 January 1872
And that his father dies not long after.
A possible marriage in Poplar district, June qtr 1877
Cornelius Noonan + Mary Sullivan.
Could Mary have been a widow?
In 1881 in Bromley there is a Cornelius and Mary Noonan family, possibly not around before, that includes Denis, 9, born Rotherhithe (piece 495 folio 26 page 45)
1881 index is free on ancestry, so I needn't write out the whole familyhttps://search.ancestry.co.uk/cgi-bin/sse.dll?_phsrc=FVe2494...
There are four children aged 9-12, including Denis. Quite a jam.
Overall, some of the names could match the Sullivan family in Rotherhithe in 1871. Is it a mixture of two families, now together by that marriage?
The family are still in Bromley in 1891 as Norman (but no Dennis with them), Cornelius, 54, and Mary, 52, both born Cork, others include Mary A, 20, and Agnes, 16.
A potential marriage for Mary A in Dec 1894, Poplar 1c 1191
Michael Finlay + Mary Ann Sullivan
(Amazingly with the same page reference is an Agnes Noonan, also Robert William Ranson! Since "our" Agnes Noonan is at home in Bromley in 1901 with Cornelius and Mary, both born Cork, we'll leave that)
Michael and Mary Finley are in Bromley in 1901, with Mary A. being 31, born Rotherhithe. Children include Dennis C
Birth reg, Sep 1900 Poplar 1c 571
Finley, Denis Cornelius - mother Sullivan
In 1891 there is a Denis Osullivan, 19, born Rotherhithe, a student at Gorton Monastery in Manchester.
Sounds unikely, but for this chap I guess anything is possible!