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Harkin/Harkins in the mid-19th century was a rare surname for Co. Longford. The surname was mostly found in Co. Donegal.
I checked the 1911 census and there were no Harkin/Harkins in Co. Longford. The online Irish phone book (which only has a percentage of phone users) shows one Harkin record in Co. Longford. http://www.eirphonebook.ie/#