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We have a parish liaison in Clondavaddog so I will contact him and alert him to your e-mail. There were likely many Mary Callahan/Callaghans born in Co. Donegal between 1846-1851. Unfortunately, most Donegal RC parishes do not have records back to the 1847-1851 period. Clondavaddog records, however, start in February 1847 http://registers.nli.ie/parishes/1011
Ballymichael is a townland in Clondavaddog parish.
Roots Ireland has four baptismal records for a John Meehan-- one in 1848 and three in 1849. You would need his parents names to confirm any of these records. Co. Mayo also has a number of parishes where records do not go back to 1847.