In March 1914, the regiment was encamped at the Curragh as part of 3rd Cavalry Brigade, which was mistakenly informed that its officers would have to impose Home Rule on Northern Ireland or resign, -in what became known as the Curragh Incident. Many of the brigade's officers were Irish Protestants and so over 80 per cent of them initially offered to resign before the mistake was corrected. This correction also allowed the 5th Lancers to sail for the Western Front five months later.
The 5th Royal Irish Lancers were one of a number of cavalry regiments that were mobilised during early August 1914 in preparation for the coming conflict with Germany. As part of 3rd Cavalry Brigade under the command of General Gough, they sailed from Dublin to France along with the other infantry, cavalry and artillery units of the British Expeditionary Force.
Their first taste of action was in the hot August of 1914 during the Battle of Mons where they fought rearguard actions in support of the retreating British forces who had been forced to abandon their positions due to a French retreat which threatened to isolate the entire British army in France.
They were present at the subsequent conflicts at Le Cateau and the Battle of the Marne which saw the end of the epic retreat from Mons and forced the Germans to turn on their heels, this time assuming the role of the pursued.
During this time, a restructuring of the Cavalry Division took place which saw 3rd, 4th and 5th Cavalry Brigades formed into 2nd Cavalry Division, which became known as Gough's Command after its commander General Gough who was previously commander of 3rd Cavalry Brigade.
In October 1914, 3rd Cavalry Brigade were instrumental in the capture of the monastery at Mont Des Cats and the fighting around Warneton and also during subsequent actions at Messines where British Cavalry units were forced off the famous ridge by German formations. This was the precursor to the Battle of Ypres or "First Ypres" as it is known. Here, along with Indian troops the 5th Lancers put up desperate resistance in their hastily dug trenches against larger German Infantry and Cavalry units.
They would spend the rest of the bitterly cold winter months of 1914 and early 1915 in these positions. This saw the start of trench warfare proper which was to set the precedent for future battles for both infantry and cavalry formations. There were to be no real mounted cavalry actions until late 1918 and the cavalry's role including that of 5th Lancers would be that of scouting actions and periods of action in the trenches instead of decisive cavalry charges that the cavalry commanders yearned for.
Much of the rest of 1915 was spent fighting at Ypres and Loos, the often neglected campaign that the British fought up until mid 1917. The 5th Royal Irish Lancers along with other cavalry formations occupied the trenches at various locations although for a lot of the time they were held in reserve.
1916 was spent mostly in reserve although again they were held in reserve for a lot of the time.
In 1917, along with the rest of 3rd Cavalry Brigade, 5th Lancers fought in such actions as Gillemont Farm and Bourlon Wood where the regiment earned their only Victoria Cross of the war. Towards the end of 1917 they fought at Cambrai.
At St. Quentin in 1918 when the Germans launched their Spring Offensive the 5th Lancers were heavily involved in the defence of the British line at the Crozat Canal and at the Canal du Nord where they suffered their heaviest casualties of the war in face of the massive German onslaught. On 11th November 1918 the regiment, along with the Canadian 3rd Division entered Mons and earned for itself the honour of being the first British regiment to enter the town from which they had been the last regiment to leave in August 1914.
The last British fatality of the war, according to the records of the Commonwealth War Graves Commission, was L/12643 Private GE Ellison, Fifth Royal Irish Lancers. He died near Mons, when a dismounted patrol of the regiment was confronted by 20 well-entrenched Germans who fired on them and then fled. In this brief exchange Ellison was shot and killed. His time of death is thought to have been 9.30am., 90 minutes before the Armistice came into effect.
It then deployed to Risalpur in India, where it was disbanded in 1921, only for a single squadron of the regiment to be re-raised the following year to merge with the 16th Queen's Lancers to form the 16th/5th Lancers.