Irish-born Costello, who joined Dai Young’s coaching team at the start of the 2018/19 season, has responsibility for defence.
Prior to joining the black-and-golds, Costello spent two seasons as head coach at Championship club Nottingham.
From 2011 to 2016, Costello was part of Munster’s coaching set-up, working initially as skills coach then as assistant coach with responsibility for defence and kicking.
Before making the move into the Thomond Park club’s Pro 12 and European Champions Cup set-up, Costello worked with their academy and U20 squads.
During this period he was head coach of the Munster A side that lifted the British and Irish Cup title in 2011.
Costello, who also has experience as director of rugby at amateur side UL Bohemians, was a fly half or centre in his playing days.