The move to the Danske Bank Premiership side marks Mastny's first temporary spell away from United since he joined the club's Academy from Vysocina Jihlava in his native Czech Republic in 2018.
The 20-year-old has made Premier League 2 appearances in each of the last three seasons and has also represented us in the FA Youth Cup, UEFA Youth League and the Papa Johns Trophy.
Mastny, who has been capped by Czech Republic at various youth levels, will now benefit from the opportunity to play senior football at Shamrock Park.
Everybody at United would like to wish Ondrej the best of luck during his time in Northern Ireland.