Peveril Street doesn't exist any more but it seems that it is currently Ormeau Road and the place where Robert Mitchell, pensioner, lived, between Lavinia and Abbott Streets (currently near McClure Street) is less than a mile from Connaught Terrace off University Street where Alfred (1877) and Mrs. Mitchell (1880) lived and it was also near the address given by Sarah Ann in her marriage certificate, 36 University Street, in 1870. I think that Alfred and Mrs. Mitchell are the right ones, not sure about Robert though, unless they had more than one property or they were separated.
Edited: I have just checked all the 1868 directory and there is no number 36 at University Street, but there was a Robert Mitchell, clerk, living at 37 Grosvernor Street.
In 1890 Mrs. Mitchell was at 1 Ivanhoe Street (Peverill Street to Abbott Street) and in 1894 at 5 Ivanhoe Street (Peveril Street to Abbott Street).
I know Elizabeth and her daughter Louisa Harriette in the 1891 census were in London at the household of her nephew Alfred Joseph Clarkson, but maybe they were just visiting?