Mulhern, 20, featured heavily for Guiseley in pre-season ahead of the 2017/18 season on trial and was rewarded a contract.
He 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 (pictured), who hails from South Elmsall, was in the Huddersfield academy for nearly four years, joining the Terriers at under-9 level in August 2005 before departing for Leeds in May 2009.
He subsequently turned professional with the Elland Road outfit in the summer of 2015 following a two-year scholarship - a deal he extended a year later.
Mulhern, who played for Southport during a brief loan spell in September 2016, has a brother Robbie who is a professional Rugby League player for Hull KR.
Weaver has also brought in Lachlan Barr from SkyBet League One Bradford City until December 19th.
Barr is an Australian-born centre-half formerly with Australian side Adelaide United.
The defender spent a period training with the Bantams last season having more recently with was FC Internationale Berlin, before signing a more permanent deal in July.