In March 1922, there was a stand-off in Limerick city between pro-Treaty and anti-Treaty units of the Irish Republican Army during the transfer of bases by the withdrawing British Army. O'Mara negotiated a resolution to this. In May 1922, O'Mara established a Limerick City Police Force to replace the withdrawn Royal Irish Constabulary. In June a general Civil War broke out across the nascent Irish Free State. The City Police Force was replaced in July by the Free State Civic Guard. O'Mara was interned by the Free State in November, but re-elected mayor in January 1923; he was released in March. He resigned as mayor in October, after the Civil War had ended in defeat for the anti-Treaty side.