In December 1921, after the signing of Anglo-Irish Treaty, Lemass was released. He became a training officer for a period in Beggars Bush Barracks before the IRA split and was involved in the Belfast Boycott operations. During the debates of the Anglo-Irish Treaty in 1921, Lemass was one of the minority who opposed it along with de Valera. As a protest, all the anti-Treaty side withdrew from the Dáil. In the Irish Civil War which followed Lemass was adjutant to Rory O'Connor, when the group seized the Four Courts, the home of the High Court of Ireland. The occupation of the Four Courts eventually resulted in the outbreak of the Civil War, when, under British pressure, the Free State side shelled the building on 28 June 1922.C Townshend, "The Republic", p.412 As a result, fighting broke out in Dublin between pro and anti Treaty factions. The Four Courts surrendered after two days bombardment, however, Lemass escaped with Ernie O'Malley and some others to Blessington.M. Hopkinson, "Green against Green", p.143-44. Their Flying Column operated in Enniscorthy, Tullow, Ferns, Baltinglass and Borris before the Column was broken up. Lemass and O'Malley returned to Dublin along with Thomas Derrig as a member of the IRA Eastern Command Headquarters but was later captured in December 1922 and interned again.
