I had a look on FreeBMD for likely births and found the following; George Sept Qtr 1869 Prescott; Mary Ann Dec. Qtr. 1865 Prescott; That is the only ones I found in Prescott, however in South Shields Co Durham there is a Francis McNally Mar Qtr 1878, and Rebecca McNally South Shields Co Durham Mar. Qtr 1876. This last one for Rebecca would have only made her 5 years old on the 1881 census, so may not be the two you are looking for.
Regards.
Mary Ann McNally b 1865, Prescot has mmn Durkan
George 1869 has mmn Murphy
Added
The two South Shields births have mmn McAffee.