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I searched on the subscription site Roots Ireland for all of Ireland for a Sarah Kelly with father Michael. No baptismal records turned up around 1850 in Co. Kildare or Co. Laois. There are 1848 and 1849 records in Co. Offaly and 1847 and 1851 records in Co. Limerick plus an 1849 Co. Down record. I assume you tried to find her mother's name with no success?