In the lead-up to the 1916 Easter Rising, a rebel plan for Galway town was prepared at Nicolls' home at 2 University Road.

He was arrested in Galway on Easter Tuesday before the local Irish Volunteers could be mobilised.

[3] In January 1922, he was appointed Assistant Minister for Home Affairs in the Government of the 2nd Dáil.

[4] Following the Anglo-Irish Treaty, he sided with Michael Collins and voted in favour of it.

In the 4th Dáil, he was appointed as Parliamentary Secretary to the Minister for Defence and served from January 1925 to May 1927.