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Frank featured heavily in pre-season ahead of the 2017/18 season on trial and was rewarded a contract.
Mulhern, joined Huddersfield Town’s Under-23 Development Squad in February 2016 from Leeds United.
Capped by the Republic of Ireland at Under-18 and Under-19 level, Mulhern, who hails from South Elmsall, was in the Huddersfield Town Academy for nearly four years joining the Terriers at Under-9 level in August 2005 before departing for Leeds United in May 2009.
He subsequently turned professional with the Elland Road club in the summer of 2015 following a two-year scholarship; a deal he extended a year later.
He played for Vanarama National League side Southport during a brief loan spell in September 2016.
His brother Robbie is a professional Rugby League player for Hull KR.