This might be a sister of your Robert, although I'm aware you're not positive who Robert's parents are (irishgenealogy.ie):

Area - DUBLIN (RC) , Parish/Church/Congregation - ST. MARY, PRO CATHEDRAL

Baptism of MARIA JEANNA SOPHIA WALL of 48 LR SACKVILLE ST on 31 October 1860

Name MARIA JEANNA SOPHIA WALL
Date of Birth 10 October 1860
Address 48 LR SACKVILLE ST
Father JOANNES DENHAM WALL
Mother MARIA JEANNA O'BRIEN
Further details in the record
Sponsor 1 ROBERTUS WALL ***
Sponsor 2 ALICIA MARIA O'BRIEN **
Priest GUILLELMUS PURCELL

*** an uncle of "your" Robert?
** a sister of Maria O'Brien?

Happy hunting!
Colette