After Bohemians, Shelbourne and St James's Gate, they became the fourth civilian team to win the league.
Shelbourne United and Rathmines Athletic originally had their applications to join the league rejected at a meeting on 17 August 1922.
However, after observing a friendly between the two teams, the league finally accepted Shelbourne United following another meeting on Monday, 4 September 1922.
Less than two weeks later, they made their League of Ireland debut on Saturday, 16 September 1922, in a 1–0 away win against Shamrock Rovers.
In the 1922–23 FAI Cup they defeated Bohemians 2–1 in the round of sixteen and in the quarter-finals they lost over two games to the eventual winners Alton United.