In 1920–21 Dublin United were playing in the
Leinster Senior League. They also won the
Leinster Senior Cup, defeating St James's Gate 1–0 in the final. In
1921–22, together with
Shelbourne,
Bohemians,
St James's Gate,
Jacobs,
Olympia,
YMCA and
Frankfort, Dublin United became founder members of the
League of Ireland. Like Dublin United, the other seven founding members had spent the 1920–21 season playing in the Leinster Senior League. During the inaugural League of Ireland season, Dublin United defeated Frankfort 6–0 and thus achieved the league's biggest home win. They eventually finished the season in seventh place. In the
1921–22 FAI Cup Dublin United defeated Frankfort 8–1 in the first round. In the second round they lost 5–1 to the eventual winners
Shamrock Rovers. In their second and last season in the League of Ireland,
1921–22, they finished in tenth place. In their final league game Dublin United were due to play
Rathmines Athletic. However Rathmines resigned from the league without playing the match. The match was later awarded as a win for Dublin United. In the
1922–23 FAI Cup Dublin United took three games to beat Sligo Celtic in the round of sixteen. In the quarter-finals they lost 3–2 to
Fordsons. Together with Olympia, Dublin United were not re-elected for the
1923–24 League of Ireland season. ==Honours==