The Start of the Williamite War In Ireland

March 12, 1689:

On 12 March 1689 James landed in Kinsale, Ireland, with 6,000 French soldiers. He first marched on Dublin, where he was well received and, with a Jacobite army of Catholics, Protestant Royalists and French, then marched north, joining the Siege of Derry on 18 April. James found himself leading a predominantly Irish Catholic movement, and on 7 May he presided over an Irish Parliament composed almost entirely of Catholic gentry. He reluctantly agreed to the Parliament's demand for an Act declaring that the Parliament of England had no right to pass laws for Ireland. He also agreed, again reluctantly, to restore to Irish Catholics the lands confiscated from their families after the Cromwellian conquest of Ireland, by confiscating the lands of those (predominantly Protestants) who opposed him and supported William. This parliament was later named the Patriot Parliament by Irish nationalists.
The Williamite War in Ireland—also called the Jacobite War in Ireland, the Williamite-Jacobite War in Ireland and in Irish as Cogadh an Dá Rí (meaning "War of the Two Kings")—was a conflict between Catholic King James II and Protestant King William of Orange over who would be King of England, Scotland and Ireland. The cause of the war was the deposition of James II as King of the Three Kingdoms by William (who was married to James' daughter Mary II) in 1688.
James was supported by the mostly-Catholic "Jacobites" in Ireland and hoped to use the country as a base to regain his Three Kingdoms. He was given military support by France to this end. For this reason, the War became part of a wider European conflict known as the Nine Years War. Some Protestants of the established Church in Ireland also fought on the side of King James.
James was opposed in Ireland by the mostly Protestant, "Williamites", concentrated in the north of the country. William landed a multi-national force in Ireland, composed of English, Scottish, Dutch, Danish and other troops, to put down Jacobite resistance. James left Ireland after a reverse at the Battle of the Boyne in 1690 and the Irish Jacobites were finally defeated after the Battle of Aughrim in 1691.
William defeated Jacobitism in Ireland and subsequent Jacobite Risings were confined to Scotland and England. However, the War was to have a lasting effect on Ireland, confirming British and Protestant rule over the country for over a century. The iconic Williamite victories of the Siege of Derry and the Battle of the Boyne are still celebrated by the Unionist community in Northern Ireland today.

This gives rise to the sectarian tensions of "marching season" to this day.