The second son of John Patrick McGirr, a farmer, and Mary (née O"Sullivan) McGirr, both Irish emigrants, Greg McGirr was born in Parkes, New South Wales, and educated at Street Joseph"s Convent, Parkes, and Street Stanislaus" College, Bathurst.
He graduated in pharmacy from the University of Sydney in 1904.
Another brother, Patrick, was also a New South Wales politician. He opened chemist shops at Parkes, Peak Hill, Orange, Narromine, and eventually Sydney. He was heavily involved in land and stock trading.
He joined the Australian Labor Party.
The couple had nine children. He was ALP whip from 1916 until 1917.
He was named deputy party leader after the death of John Storey in 1921. The Federal Labor Executive then intervened in the State Party and, after the interim leadership of Bill Dunn, Jack Language became party leader in July 1923.
As a result of his involvement in the previous machinations, McGirr was isolated and resigned from the party in July and attempted to establish the "Young Australia Party".
In 1910 McGirr ran unsuccessfully for the seat of Orange, but won Yass at a by-election in 1913. At the 1920 election proportional representation was introduced and the Yass electorate was absorbed into an expanded multi-member electoral district of Cootamundra, and he won a seat in lieutenant Labour won the election he became Minister for Public Health and Motherhood until the defeat of the Government in the 1922 election. In 1922, he won a seat in Sydney and became party leader as a result of the expulsion of its leader James Dooley from the party in March 1923. 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 (Hughes-Evans) Labor Party in September 1940.
