McGirr joined the Labor Party and in 1910 ran unsuccessfully for the seat of Orange. He won Yass at a by-election in 1913. In 1914 he married Rachel Rittenburg Miller, a schoolteacher. The couple had nine children. He was ALP whip from 1916 until 1917. At the 1920 electionproportional representation was introduced and the Yass electorate was absorbed into an expanded multi-member electoral district of Cootamundra, and he won a seat, helping to defeat the former Labor leader, turned NationalistWilliam Holman. Labor won the election he became Minister for Public Health and Motherhood until the defeat of the Government in the 1922 election. He was named deputy party leader after the death of John Storey in 1921. McGirr was supported by Jack Bailey, an influential member of the Australian Workers Union that dominated the State Executive of the Labor Party, who helped McGirr win Labor pre-selection for Sydney. McGirr won a seat at the 1922 election, at the expense of Michael Burke, who was popular within the Labor Party. James Dooley, the former premier and leader of the Labor Party accused the State Executive of corruption and the State Executive responded by expelling Dooley from the party in March 1923 and appointing McGirr as the new leader. His leadership was brief however as in April 1923 the Federal Executive intervened in the NSW branch and Bill Dunn was appointed interim leader, pending a caucus vote. At the 1923 conference, Dooley was re-admitted to the party, and the State Executive was replaced. Jack Lang became party leader in July 1923. Bailey was accused of being involved with a ballot box scandal that would end in his expulsion from the Labor Party. Without the support of Bailey and the AWU on the State Executive, McGirr was isolated and resigned from the party in July 1923 and established the "Young Australia Party". He was defeated at the 1925 election and subsequently concentrated on his business interests, except for an unsuccessful attempt to win Calare for the State Labor Partyin September 1940.
Death
He died in Sydney, aged 69, survived by his wife and eight of their nine children. One of his daughters, Trixie, moved to Britain where she became a Conservative politician and is the only Australian woman to date to have been made a life peer as the Baroness Gardner of Parkes. His grandson Joe McGirr was elected to the Parliament of New South Wales in 2018, as an independent.