Both players' current deals expire during the summer and they do not figure in manager Greg Abbott's plans.
Abbott said: "Both lads have worked hard and they've been a pleasure to have around.
"They've both been unlucky with appearances this season because of the way the team has performed and they leave here with our very best wishes."
Curran, 22, signed a two-year deal in the summer of 2010 and scored eight goals in 69 appearances for the Cumbrians. He spent the last month of this season on loan at Morecambe, where he scored once in seven appearances.
O'Halloran, 24, was handed a one-year contract last summer when he arrived from Coventry and has been restricted to just five appearances.