Doing research on Uí hEarcáin: an Ancient Irish Sept
The surname Uí hEarcáin is plural transliterating in English to Harkins, while ÓhEarcáin and Ua hEarcáin(Ní hEarcáin being the feminine form) transliterate to Harkin; HOWEVER, both are the same extended family which in Irish is "Sept" as opposed to Clann which is a group of (different) families led by a common (usually war)leader.
That said I would recommend that those researching their family's origin examine both the Harkins, Harkan and Harkin spellings of this surname.