The garrison of Ireland before the Commonwealth consisted of independent companies of foot and troops of horse, and towards the end of the reign of Charles II they were formed into battalions; but of the infantry regiments so raised only the Royal Irish remains.  Composed at first largely of Roman Catholics, these were discharged when William III became king, and the reorganised regiment served in Ireland against James II, and shared in the siege of Carrickfergus, at the Boyne, Limerick, Ballymore, Athlone, and Aughrim; after which it returned to England, and embarked for Flanders, serving also as marines in the fleet,  Returning to the Netherlands in 1694, it was present at the siege of Huy, and after much arduous service, behaved with the greatest gallantry at the siege of Namur.  There, in the attempt to storm the breach, the 18th lost 26 officers and 261 men, killed and wounded, for which the King rewarded the regiment by granting as a badge to the colours the lion of Nassau, the harp in a blue field, crowned, and the motto "Virtius Namurcensis Praemium".  From 1701 until 1714, with the exception of a shirt absence, the Royal Irish served on the Continent, and shared in almost all the principal combats.  Beginning with the siege of Kaiserswerth. it was at Nimeguen, Venloo, Fort St Michael, Ruremonde, Liege (where a quantity of dollars was looted), Huy, Lemberg, Schellenberg, Raiyn, Belnheim (where the loss was 12 officers and 143 men), and Malplaquet, when the Royal Irish Regiment of England met and defeated the Royal Irish Regiment of France.  It also served at Douay, Bouchain, etc.  After Fontenoy in 1745, the regiment again returned to Flanders, but was recalled because of the rebellion of 1745, and it saw no further service until 1775, when it fought at Lexington and Bunker's Hill.  The year following it was removed to Nova Scotia.

In 1783 it was at Gibraltar, whence an expedition was successfully made against Toulon in 1793; and on the Mediterranean station it practically served until 1802, seeing service in Corsica at Calvi; in Italy at Campiglia, Castiglione and Piombino; at Minorca, and Genoa; and finally in Egypt at Aboukir, Alexandria, Fort St Julian, El Aft, Rahmanie, and Cairo, for which the Royal Irish was authorised to bear on its colours the Sphinx and "Egypt".

After a short service in the West Indies, at St Domingo, and Jamaica (where the loss from disease in twelve years was 50 officers and 3,000 men), it sailed in 1837 to Ceylon, and in 1840 formed part of the army in the first China War, being engaged at Chusan, Tinghae-hien, Canton, Amoy, Koolangso, Chinhae, Ningpo, Tseekee, Segaon, Chankee Pass, Chefoo, Woosung, Shanghai, Chin Kiang, and Nankin.  For these services the Draton and "China" are borne on the colours.  It is related that at the attack on Amoy the Chinese were greatly annoyed at the great battery being stormed from a flank, instead of in front, as such a course "was not fair fighting"; and it was also said that the "Tiger-Braves", so called from their striped uniform, attempted by clattering their wicker shields and jumping in the air to frighten them off "on the wing".  The regiment next took part in the Burma War of 1851 - 52, earning the badge "Pegu"; and, transferred to the Crimea in 1854, it did good work during the siege of Sevastopol, especially in the fighting at the Cemetery on the 18th June, 1855.

The second battalion was first formed in 1803, but this disappeared in 1814.  The present 2nd battalion was formed in 1858, of volunteers from the Irish Militia.  The original name of the regiment was the Royal Regiment of Foot of Ireland, but this was altered to its present title, the Royal Irish Regiment, as far back as 1695.  Its seniority in number depended on the date when it was first placed on the English establishment; but its place, taking its total service into consideration, would be the 5th or 6th of the Line.

The 2nd battalion formed part of the force in New Zealand in the campaign of 1863 - 66; while the first battalion furnished a brilliant regimental record by adding "Afghanistan, 1879 - 80", to the list of honours (for service on the line of communications through the Khyber Pass), and "Egypt, 1882", "Tel-el-Kebir", and "Nile, 1884-5", for its prominent services in that campaign.  At the first mentioned battle the men went "straight at the works", and were specially mentioned in Lord Wolseley's despatch; and they were the first up the Nile in the campaign for the relief of Gordon, winning the prize of a silver boat, which the commander of the expedition had offered for friendly competition among the troops.

The button bears the harp within a circle, and the words "Virtutis Namurcensis Praemium", with a shamrock wreath; the collar has the arms of Nassau and motto; the harp, crown, and shamrock also appear on the helmet plate and waist plate.  The gold lace, too, is of shamrock pattern.  The <Militia battalions are the Wexford, North Tipperary, and Kilkenny regiments.  The regiment's only name is the Royal Irish, though it has, when in Ireland, been also named the 18th Native Infantry.  The depot is at Clonmel.