In October 1941, during World War II, the two militia battalions were delinked and the 55th was later deployed to New Guinea, where they took part in the Kokoda Track campaign, fighting against the Japanese.

The 55th Battalion was initially raised during World War I in February 1916 as part of the expansion of the Australian Imperial Force (AIF) that was undertaken in Egypt after the end of the Gallipoli campaign.

[3] At this time the size of the AIF was doubled by forming new battalions from reinforcements from Australia and experienced personnel drawn from units that had served at Gallipoli.

[5] The battalion's final involvement in the fighting came late in September when they took part in the Battle of St. Quentin Canal, which resulted in Private John Ryan being awarded the Victoria Cross (VC).

[5][Note 2] The instance of the third Bar to the MM is unique, and was bestowed upon one of the 55th Battalion's stretcher bearers, Private Ernest Albert Corey.

[12] In 1927, when territorial designations were adopted, it became the "New South Wales Irish Rifles"; the battalion's motto, Faugh-a-Ballagh was approved at this time too.

In 1928, a feature film was released which had been made by two members of the battalion, The Spirit of Gallipoli, promoting the role of the army in peacetime.

[14][15] In 1929, after newly elected Scullin Labor government decided to suspend the compulsory training scheme and replace it with a voluntary system.

[20] In October 1941, as tensions in the Pacific grew, the 55th/53rd Battalion, as part of the 14th Brigade, was undertaking a period of continuous training at Bathurst, New South Wales, when it was announced that they were to be delinked once more.

The following month the 55th Battalion was re-raised in its own right and after moving to Greta Camp it received fresh drafts of reinforcements and was subsequently brought up to its allocated establishment.

In December, following the Japanese attacks on Pearl Harbor and Malaya, the 55th Battalion was transported to Newcastle, New South Wales, where they set up defences along the coast in the Raymond Terrace–Stockton Beach area.

[13] The following year, as the strategic outlook deteriorated following the fall of Singapore, the battalion was warned to prepare to move and over the course of a week between 22 and 29 May 1942 they were transported to Port Moresby in New Guinea.