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I searched the subscription site Roots Ireland and found baptismal records for the seven children you listed plus a record for a Pat in March 1851. Which is strange since Bridget was born in June 1851. So there are at least two possiblities, there was another John and Peggy McGowan couple in the parish or the year was mistranscribed and it was actually 1850. As I'm sure you know, the parish register is not very legible and possibly Cornelius' record is there but very hard to find. Do you have a death record for Cornelius which might list his parents names?
The other complication is the great number of McGowan families in Rossinver civil parish. There were 219 McGowan families in the parish in 1901.
Name:Pat MacgowanDate of Birth:
Date of Baptism:23-Mar-1851Address:
Parish/District:KINLOUGH ROMAN CATHOLICGender:MaleCountyCo. Leitrim
Father:John MacgowanMother:Peggy MacgowanOccupation: