27th Regiment gains 2nd Battalion.

In April 1800, the Inniskillings moved to Dover Castle, where, on 25 May, for the first time in their history, they became a two-battalion regiment.

The 2nd Battalion was composed mainly of volunteers from Irish Militia regiments, two of which were the Donegals and the Fermanaghs (later in 1908, titled the 5th and the 4th Battalions of the Royal Inniskilling Fusiliers respectively), providing between them 143 out of the 543 recruits raised in Ireland; there were also recruits from England. At the end of July 1800, both Battalions were attending a training camp in Windsor Forest when, unexpectedly, they were warned for overseas service in the Mediterranean.

As a consequence of the post-Napoleonic War economy measures of 1817, the 2nd Battalion was disbanded on 24 May 1817. Three hundred men transferred to the 1st Battalion, then stationed in Cork. In the seventeen years of its existence, the 2nd Battalion earned a reputation of distinction.