The Royal Irish Fusiliers was increased to 14 battalions, most of these fought on the Western Front,others fought in Gallipoli, Palestine, the Balkans and Egypt. Battalions from the Royal Irish Fusiliers fought with the 10th and 16th Divisions and the 36th Ulster Division. There were also Garrison Battalions in Ireland, whose task it was to recruit and train the soldiers ready for fighting on the front line.
The 1st Battalion fought at Le Chateau and shared in the battle of Maine and advance to the Aisne. It was involved in the capture of Armentieres and in the second battle of Ypres 1915, where it suffered two of the earliest gas attacks in the war.
The Battalion took part in the battle of the Somme, 1916 and the battle of Amas 1917. Also in 1917 the Battalion became part of the 36th Ulster Division, fighting with distinction in the battle of Cambrai. As a result of the German offensive in March 1918, the Battalion suffered heavily and was reformed into one company attached for a time to the 9th Battalion. When it was restored to strength, it took part in the third battle of Ypres 1918. Private Robert Morrow of the 1st Battalion was awarded the Victoria Cross for conspicuous bravery near Messines on 12th April 1915. During the war 1,058 men died while serving with the 1st Battalion.
The 9th Battalion was formed in 1914 and became part of the 36th Ulster Division. On the first day of the battle of the Somme in 1916 this battalion attacked Hamel and suffered terrible casualties, with 240 men killed in one day. The Victoria Cross was awarded posthumously to Lieutenant Geoffrey St George Shillington Cather of the 9th Battalion for rescuing wounded men during the battle of the Somme on 1st July 1916. The battalion fought at the battle of Messines 1917, the third battle of Ypres 1917, St Quentin, Lys, Kemmel and Coutrai 1918. In 1917 the badly hit North Irish Horse joined the ranks.
The 11th Battalion went to France in 1917, but due to severe losses was soon absorbed into the 5th Battalion. During the war the Regiment won 44 Battle Honours, over twice as many as had been won in all its previous service. The cost was high, the regiment having lost 3,181 dead and more than 15,000 wounded. By 1922, civil war and partition in Ireland forced the disbandment of many Irish infantry regiments, leaving only the Royal Inniskilling Fusiliers and the Royal Irish Rifles. The Royal Irish Fusiliers were to be disbanded, but were saved by the generosity of the Inniskillings who reduced to a single battalion, making room for them.