[1][2][3] During the 1940s Castlebar Celtic began to compete in provincial competitions such as the Western League and the Connacht Junior Cup.

[3][4] In 1954, together with Westport Town, Barcastle and Quay Hearts, Castlebar Celtic became founder members of the Mayo Association Football League.

[5] They finished the inaugural season as champions and together with Westport United, Castlebar Celtic became one of the league's two most successful clubs.

[4][6] In 1987–88 Castlebar Celtic joined the Connacht Senior League and in their first season finished as runners up behind Galway United Reserves.

The game was rescheduled after an assault on the referee by a Salthill Devon fan caused the original fixture to be abandoned with Celtic leading 1–0.

Castlebar Celtic were invited to participate because reserve teams were ineligible to compete in the League of Ireland Cup.

The club was encouraged to make the move from provincial to national level after winning the 2006–07 FAI Youth Cup.