Not sure if we have this.
1911 census has Beccy Cooper aged 29 in 33 Mabbot Street with her husband William Whittaker, refreshment room keeper aged 31, their children Rebecca (6) and Marey E (2), her sister Norah (22) and Norah's daughter Sarah (3).
Only Sarah Cooper possibilities I've found being born at the rightish time are one in Taghmon, Wexford, whose mother is Kate; and Mary Cooper and also Albert Cooper, both born in the South Dublin Union workhouse in 1905 and 1903 respectively, whose mother was Mary Cooper. (These are long shots so I won't attach them.)
However, I did find Rebecca Cooper, daughter of Beccy and William - who is described on her birth registration as a sergeant of the Royal Dublin Fusiliers in 1905, when they were living in 9 Railway Street.