The Battle of Rathmines was fought between the Irish Confederate-Royalist army of James Butler, Marquess of Ormond and the Parliamentarian garrison of Dublin under Michael Jones. The Parliamentarian victory at Rathmines was the decisive battle of the war in Ireland, as it enabled Oliver Cromwell to land in Dublin with 12,000 troops shortly after, beginning the Cromwellian conquest of Ireland.
In September 1643, King Charles I's commander in Ireland, the Marquess of Ormond, agreed to a "cessation" with the Irish Confederation, freeing up his 5,000 veterans for use in the First English Civil War. Even after the end of the civil war in 1646, Charles continued secret negotiations with the Confederation for military support. In June 1647, Ormond handed over Dublin to the Parliamentarians rather than see the city lost to the Confederates. He proceeded to enter exile as Jones won a series of victories in Leinster, preventing Irish intervention in the Second English Civil War.
Ormond returned to Ireland in October 1648 and allied with the Confederates in 17 January 1649, planning to restore the deposed Charles to power. Ormond's army of Anglo-Irish Protestants, English Catholic exiles, and a small number of Irish Catholics recruited local Protestants (among them deported English Royalists), and, on Charles' execution on 30 January, they were joined by Ulster Presbyterians who saw the monarchy as divinely ordained and his execution as sacrilegious. By the end of May 1649, the Royalist-Confederate alliance controlled most of Ireland. After taking Drogheda and Dundalk in June, Ormond besieged Dublin, held by Jones' 3,000 men and now encircled by 11,000 troops.
However, the Parliamentarian admiral Robert Blake blockaded Prince Rupert of the Rhine's Royalist naval squadron at Kinsale, preventing them from aiding in the siege. On 26 July, Robert Venables reinforced Dublin with 4,000 veteran infantry and 1,200 cavalry. As Oliver Cromwell and 9,000 more Parliamentarian troops prepared to set sail from Bristol, the Royalists and Confederates planned an assault on Dublin.
Jones immediately attacked as Major-General Patrick Purcell marched to occupy Baggotrath Castle, scattering Purcell's men and killing Sir William Vaughan. Jones' cavalry outflanked the Royalists and captured their artillery train, and Ormond's left disintegrated without firing a shot. The Royalist-Confederate army was forced to retreat to Drogheda, and hundreds were cut down in the pursuit; Christopher Plunket, 2nd Earl of Fingall was mortally wounded and captured. 300 of Ormond's men were allegedly shot after surrendering. The Parliamentarian victory enabled them to create a defensive line covering the road between Dublin and Ringsend, where Cromwell landed on 15 August. The divided and demoralized Protestant Royalists deserted in largef numbers over the next few months.