Donnellan, who was born in Barnet, started his career at West Bromwich Albion but never forced his way into their first-team plans as a youngster.
He had various loan spells away from the Hawthorns to gain first-team experience.
These included spells at Worcester City, Stevenage, Dagenham and Redbridge and Walsall.
The young centre-back spent the first part of this season on loan at Walsall in League One.
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