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Can you clarify how you know that the 1813 record is your ancestor? The reason I ask is that there is also an 1818 baptismal record for a Margaret McDonnell with parents Cors McDonnell and Mary Sullivan. This means that there were two couples with the same names in the sam parish or the 1813 Margaret died and the couple named a later child Margaret. This was a fairly common custom. I'm using www.irishgenealogy.ie as a source for records for parishes in the Diocese of Cork and Ross. Other parishes in the Diocese of Cloyne in Co. Cork are found on the subscription site Roots Ireland.
Area - CORK & ROSS (RC) , Parish/Church/Congregation - CORK - SOUTH PARISH
Baptism of MARGARET MCDONNELL of N/R on 29 December 1818
NameMARGARET MCDONNELLDate of BirthN/R N/R N/RAddressN/RFatherCORS MCDONNELLMotherMARY SULLIVAN
Sponsor 1JOHN MURPHYSponsor 2BRIDGET MURLEY