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Below is a link to the parish register for Clonleigh/Lifford RC Parish. Unfortunately, there is a big gap in baptismal records from May 1837 to early 1853 and most of the Gallagher children were born in that period. Roots Ireland did have a record for Ellen in 1837 and Alexander in 1854.
I also checked the 1857 Griffiths Valuation head of household listing for Clonleigh civil parish and there was no Gallagher in Cavanacor which is unfortunate because you could have tracked the location of the property. Either Edward had a very small holding or possibly he was deceased by 1857.
I checked the 1901 census for Cavanacor and there were no Gallagher’s listed.
I also checked the 1826 Tithe listings for Cavanacor and only the landlord was shown.
You may want to consider autosomal DNA testing.
By the way, my Brennan ancestors also lived in Clonleigh parish and most of them went to Philadelphia.