If Ellen was born in the Lismore area, and her birth was registered, then the best way to narrow down the possible births is by checking the registration district on the BMD index. The town of Lismore is in the centre on the registration district of the same name. Details on parents will only be included on the birth certs, not the index.
It's could be very difficult to locate her marriage without some details on who she married, or where ..
p.s have you located Ellen and/or her parents on the 1911 census ?
Shane