The design of these ships were developments of earlier British battleships: they mounted more powerful 40-calibre 12-inch (305 mm) guns and featured heavier armour protection, all while maintaining the preceding Canopus class' top speed of 19 knots (35 km/h; 22 mph).

After operations with the Dover Patrol, during which she bombarded German forces in northern France, she was assigned to the Dardanelles Campaign in February 1915.

Formidable adopted the larger size of the Majestics, while taking the stronger Krupp armour of the Canopus design.

The Formidable-class ships were powered by a pair of 3-cylinder triple-expansion engines, with steam provided by twenty Belleville boilers.

[2] The keel for HMS Irresistible was laid down at Chatham Dockyard on 11 April 1898 and launched on 15 December 1898 in a very incomplete state to clear the building ways for the construction of the battleship Venerable.

[3] The ship was commissioned at Chatham Dockyard on 4 February 1902 by Captain George Morris Henderson and a complement of 870 officers and men for Mediterranean Fleet service.

[8] She suffered two mishaps during her years in the Mediterranean, colliding with the Norwegian merchant steamer SS Clive while steaming in fog on her way to her commissioning on 3 March 1902, sustaining considerable damage to the side of her hull, and running aground at Malta on 5 October 1905.

Her duties included bombardment of German Army forces along the Belgian coast in support of Allied troops fighting on the front.

There, the British and French fleets, under the overall command of British Admiral Sackville Carden, were preparing to launch a major attack on the Dardanelles strait; the Entente commanders hoped to force the Dardanelles and enter the Sea of Marmara, where they could attack the capital of the Ottoman Empire, Constantinople, directly.

By clearing these fields, Allied warships could now enter the Dardanelles themselves, opening the route to attack additional fortifications around the town of Dardanus.

While other vessels shelled the forts there, Irresistible and the battleship Vengeance sent men ashore to destroy an abandoned artillery battery near Kumkale, with both ships remaining off shore to support the raid.

The 75-man team from Irresistible landed unopposed at Sedd el Bahr, but quickly came under attack from a superior Ottoman force and was compelled to retreat.

6-inch fire from Irresistible broke up the Ottoman attack, however, and the landing party was able to make their way to the fort, where they found four of the six heavy guns still operable.

[12][16] Two days later, Irresistible repeated the operation with another 78-man landing party under the command of Lieutenant F. H. Sandford, again at Sedd el Bahr.

The following day, Admiral John de Robeck, the British fleet commander, transferred his flag from Vengeance to Irresistible, while the latter was in Mudros for repairs.

[12][17] On 3 March, another attack was launched against the fortresses, and Irresistible contributed a reconnaissance party that went ashore under cover of a heavy bombardment from the fleet's battleships.

The next morning, Irresistible, the battleship Cornwallis, the light cruiser Dublin, and the seaplane carrier Ark Royal made another foray to attack targets of opportunity.

In response, de Robeck, who had succeeded Carden as the commander of the Dardanelles campaign, proposed to launch a major daylight attack on the fortresses, suppress them as best as possible at longer range, and to clear the minefields at the same time.

[20] At 15:14, Irresistible was rocked by an explosion, and by 15:32, had begun to take on a list, prompting de Robeck to order her to withdraw to avoid further damage.

Her list had increased and the fire from the Ottoman guns had become very heavy, so the remaining men were evacuated and Ocean began to withdraw.

[12][21] With the heavy losses that day—Bouvet, Irresistible, and Ocean, which had been mined and disabled after withdrawing from her attempt to rescue Irresistible—de Robeck issued the order to break off the attack.

[22] That evening, the destroyer Jed entered the Dardanelles to torpedo and sink the two abandoned battleships to prevent their capture in case they had remained afloat, but could find no sign of them.