The regiment was formed in 1684 by the Earl of Granard from independent companies in Ireland. In 1695, the regiment became known as the Royal Regiment of Ireland due to its performance at Namur under the direction of King William III. The regiment also won the right to display the King's arms on their colours along with the harp and crown. The regiment served throughout the turn of the 18th century in continental battles before being sent to Gibraltar. In 1751, the regiment was officially ranked as the 18th Regiment of Foot - although it was older than all but six other line regiments, it had not been placed on the English establishment until 1689, lowering its precedence.

The regiment was in Ireland during the majority of the Seven Years War and was ordered to America on 1 January 1767. The regiment arrived at Philadelphia on 11 July 1767 under the command of Lieutenant Colonel John Wilkins. The regiment remained at Philadelphia, although a small detachment was sent to Ft. Pitt later that summer. The majority of the regiment under Wilkins was ordered to Illinois in early 1768 and remained in Illinois until April 1772 when Fort Chartres was abandoned. A small detachment under Captain Hugh Lord, remained at Ft. Gage (Kaskaskia), Illinois until May 1776 when it was ordered to Detroit in anticipation of an American attack. The rest of the regiment was present in Boston, where the grenadier company participated in the Battle of Lexington and Battle of Concord and Bunker Hill, its first formal combat in more than 50 years. The regiment was drafted into other regiments in Boston in December 1775 and at Detroit in July 1776. According to the regimental History the losses at Lexington/Concord were 2 killed/4 wounded {.p. 48} and at Bunker Hill 3 rank & file killed while Lt Richardson and seven rank & file were wounded {.p. 49} Lord's Detachment was drafted into the 8th (King's) Regiment in July 1776.

The Royal Irish returned to Gibraltar in 1783, where they remained until the Siege of Toulon in 1793. The regiment's second battalion, formed mainly from volunteers from the Irish Militia, began to arrive in New Zealand from 4 July 1863. It served in the Waikato and Taranaki Campaigns. Captain Hugh Shaw won the V.C. when he rescued wounded soldiers during a skirmish at Nukumaru near Wanganui. The battalion was the last Imperial Army unit to leave New Zealand in February 1870.