[2] Shelbourne began the 2021 season with a 0–0 draw away to Galway United; Georgie Poynton, Sean Quinn and Ryan Brennan were the only surviving members of the 2020 squad in the starting eleven, with ex-Cork City defender Kevin O'Connor standing in as captain.

New signing Glen McAuley received a red card in the 41st minute for a dangerous challenge on Maurice Nugent in a game which saw 9 players make their Shels debuts.

[7] Sean Quinn and Glen McAuley departed the club in July, while Eric Molloy, Kameron Ledwidge and Yassine En-Neyah arrived as Shelbourne looked to strengthen their squad ahead of the final ten games.

[8][9][10][11] Molloy made his debut as a substitute as Shels exited the FAI Cup in the first round via a 0–2 defeat away to UCD[12] Due to squad absences a much changed team lined up against UCD in the league on 7 August 2021, with Under-19's goalkeeper Jack McCarthy making his debut and keeping a clean sheet after Brendan Clark and Jack Brady were unavailable for selection.

Ex-UCD striker Yoyo Mahdy replaced Brian McManus late on and scored an equaliser in the 88th minute to preserve Shels unbeaten home record for the 2021 season.