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Doon is a civil parish and RC parish in the northeast corner of Co. Limerick on the border with Co. Tipperary. Records for Doon RC parish start in 1824, a little too late for your interests. The Church of Ireland records go back to 1804 but my guess is that this Margaret Kennedy was RC.
I did search Roots Ireland and there were no Margaret Kennedy baptismal records in either Doon RC or C of I around 1800-1820.
I assume you contacted your macth to see what she knows about her family?