The 24-year-old joins the Robins from Southern Premier Division Central side Alvechurch, for whom he has made 15 appearances this season.
Hallahan signed a professional contract with League One side Burton Albion after graduating from their academy before going on loan to Belper Town.
He made the move to Southern Premier Central club Hednesford Town in November 2018 after a spell with Coalville Town and made 15 appearances, scoring once in 2018/19.
A Republic of Ireland under-19 international, he switched to league rivals Bromsgrove Sporting in May 2021 and then to another, Alvechurch, in November 2021.
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