I had a good look through School Lane in 1851 but couldn't find anything.
I accidently stumbled across Thomas Street which I didn't realise was so close by and found the following:-
Page 49 of 81 St. George - ln (LN)
10CT 2H Thomas St.
William Devitt 32 Labourer - Ireland
wife Bridget 30 also Ireland
Page 52 of 81 St. George St. - ln
53 Thomas St.
Jane (?) Devitt 50 - mother-in-law (I couldn't read her occupation) - Ireland
Head of House was illegible - possibly William Ward
Ellenor his wife (ní Devitt) 24 - Ireland
Now if Mary Devitt resided in Thomas St. prior to her marriage, there is a good chance that William Devitt is her brother or that Jane (?) Devitt is her mother and Ellenor her sister. Or possibly both of these.
None of these people seem to appear on an ordinary search on Ancestry. I may just be over tired. Have you seen the time

Goodnight
Tony
additional information posted 12/01/2008
I bought the marriage cert. of William Ward to Ellen Devitt 07/05/1846.
Ellen's father is Charles Devitt, Shoe maker. They are living in Seel Street.
This only goes to prove that Ellen is not Mary's sister - but she may be a cousin.
I found this on the 1841 census
There is a posssibility for her family (without her) - Redmond Place (nr. Circus St. which was off Byrom St.). Her father is James, 35 b. Ireland (and as on Mary's marriage cert. he is also a Mason), Wife Marjory 32, their children Charles 10, Ellen 8, John 2, James 4mo.(all Liverpool born).
I need to find Mary Devitt by herself in 1841 aged about 15 -17 - possibly In Service somewhere.
Having a child Charles gives a possibility that James Devitt may be the brother of Charles Devitt, father of Ellen.