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I live in Kilbannon the parish where Mullagh is situated. I have never heard of Ballyduggan but perhaps it is supposed to be 'Ballygaddy' as they are quite close to each other. (I have just called somone who knows the area but he has never heard of it!)
The only baptism for an Ellen Duggan born in Galway in 1885 I could see was in Lisheenavalla in the parish of Lackagh to parents Manus Duggan and Honor Bourke. Can you let me know where you found the birth date and the reference to Mullagh?