Dennis Lacey, better known as 'Dinny',(1890–1923) was an Irish Republican Army officer during the Irish War of Independence and anti-Treaty IRA officer during the Irish Civil War. Lacey was born in 1890 in a village called Attybrack, near Annacarty, county Tipperary.
He joined the Irish Volunteers in 1913 and was sworn into the secretive Irish Republican Brotherhood in 1914. He was introduced to the IRB by Seán Treacy. During the War of Independence (1919–1921) he commanded an IRA flying column of the 3rd Tipperary Brigade. In July 1920, this guerrilla unit mounted two successful ambushes of British forces - killing six British soldiers at Thomastown near Golden, County Tipperary, and four Royal Irish Constabulary men at Lisnagaul in the Glen of Aherlow.
In April 1921, following another ambush of British troops near Clogheen, he captured RIC inspector Gilbert Potter, whom he later executed in reprisal of the British hanging of republican prisoners. In December 1921, his unit split over the Anglo-Irish Treaty. Lacey opposed the Treaty and most of his men followed suit. He later commanded the anti-Treaty IRA's Second Southern Division.
In the ensuing civil war (June 1922-May 1923), he organised guerrilla activity in north county Tipperary against Irish Free State (pro-Treaty) forces. He was killed in an action with Free State troops at Ballydavid, near Bansha in the Glen of Aherlow on February 18, 1923. Over 1,000 Free State troops, under the command of General John T. Prout, with the intention of breaking up his guerrilla unit, converged on the Glen where he and four other men from his column were billeted. Lacey and one of his men were killed and others captured. Two National Army soldiers were killed in the action.