Thomas Francis O'Byrne was born at Westbury, near Launceston, Tasmania, on 30 August 1871, the son of Irish Catholic immigrants Ellen Esther Ryan and her husband, Thomas O'Byrne, a farmer. Young Tom attended Cluan public school, drove a bullock team on the family farm, and later worked at the Mt Bischoff tin mines. A big, strapping lad, he excelled at athletics, Australian rules football and boxing: he is said to have fought the Tasmanian heavyweight champion at the age of 16.
In 1893 O'Byrne arrived at Campbelltown (Bluff), Southland, New Zealand. He carried his swag in search of work and spent eight months as a farmhand at Gummies Bush, north of Riverton. He then took up work as a bullock driver and tramwayman at H. A. Massey's sawmill at Woodend, near Invercargill. There he met Joanna (Annie) O'Brien, the daughter of a local settler; they were married on 31 December 1894 at St Mary's Catholic Church, Invercargill. O'Byrne worked at Woodend for more than 10 years, leased a small farm there, and became a prominent member of the local community.
Working conditions were harsh in Southland's scattered and often isolated sawmills. Accidents were common, wages low, hours long, and accommodation for workers and their families was usually primitive. The Southland Timber-yards and Sawmills Union was established at Woodend in December 1900. A founding member, O'Byrne was elected vice president in 1902 and president in December 1903. By 1905 he was working as bush manager for the New Zealand Pine Company, but in June 1906 he became the union's full-time organising secretary. He immediately toured the remote mills of western Southland, and enrolled 352 new members in six months. By 1910, with over 900 members working at about 75 mills, the Southland Timber-yards and Sawmills Union was the largest timber workers' union in New Zealand.
A moderate organisation, the union sought improved accommodation, the provision of first-aid equipment at sawmills, regular inspection of machinery, and modest wage increases. O'Byrne supported railway expansion and reafforestation, opposed timber imports, and advocated state ownership of the industry. He represented timber workers on the revived Southland Trades and Labour Council from March 1907, and was secretary of the Southland and Otago Cheese-factory Managers' and the Southland Dairy Assistants' unions.
Tall and immensely strong, O'Byrne's commanding physical presence helped him to earn the respect and loyalty of bushmen, who valued manliness highly. An accomplished axeman, he helped to organise the annual Invercargill axemen's carnival; he was also vice president of the Southland Athletic Association, and a lock forward for the Britannia Football Club. His powerful fist proved handy in union work: a disgruntled worker once attacked O'Byrne with a logging hook – 'A right hook settled him'; another offered to fight the secretary to avoid paying his union fees – 'He had to pay his sub,' recalled Tom. Photographs of O'Byrne posing in boxing gloves, with captions such as 'Tom O'Byrne enforces the new award,' reinforced his reputation as a tough, uncompromising unionist. But he was no militant: he supported the arbitration system and considered strikes to be a weapon of last resort, while his genial personality made him popular with workers and employers alike.
Although he supported industrial unionism, O'Byrne stressed 'the necessity for keen, intelligent interest in politics on the part of our workers.' In 1904 he had been president of the Southland branch of the Liberal and Labour Federation of New Zealand and a national vice president, but he soon quit the Liberals for independent labour politics. He was an executive member of the United Labour Party of New Zealand in 1912–13, was active in the Social Democratic Party (1913–16), and supported the formation of the New Zealand Labour Party in 1916. Although he lacked the speaking skills to become a major national figure, O'Byrne played an important role as a mediator between militant industrial unionists and the cautious leaders of the trades and labour councils. He also ensured the allegiance of timber workers and other Southland unionists to the successive labour parties. He served on the Invercargill Borough Council from 1913 to 1917.
Tom O'Byrne supported New Zealand's effort during the First World War, but opposed military conscription. Bush workers were enthusiastic volunteers: by 1917 over 500 members of the Southland union had enlisted, causing a severe labour shortage. In 1918 O'Byrne visited the West Coast and organised a timber workers' union with 400 members. Two years later he helped Auckland timber workers negotiate their award. In 1923 he travelled over 7,000 miles on organising tours. He campaigned to improve the safety of bush tramways and joined with mill owners to demand protective tariffs against cheap imported timber from North America. In 1927 O'Byrne was re-elected to the Invercargill Borough Council (he served until 1931, and again from 1933 to 1935).
By the late 1920s the timber industry was in decline, and conditions worsened with the onset of the depression. In 1931 O'Byrne reported that two-thirds of Southland's mills had closed, while the remainder were working at half their capacity. The union's membership slumped from 1,050 in 1926 to 170 by December 1931, and it almost succumbed. Prospects brightened in 1933, however, and the following year membership climbed to over 400. In June 1934 George Forbes's coalition government appointed O'Byrne to the Legislative Council. He sat on the council's labour bills committee and took a keen interest in issues concerning forestry, arbitration and workers' compensation. Although he rarely spoke, O'Byrne was a popular member and remained on the council until its abolition in 1950.
The same year, after 44 years in the job, O'Byrne retired as secretary of the Southland timber workers' union (by then a branch of the powerful New Zealand Timber Workers' Union). He died in Invercargill on 30 September 1952, survived by three daughters and a son; Annie had died in 1941. Tom O'Byrne's 'perennial smile' and love of anecdote made him many friends, and it was often said that he knew everyone in Invercargill. The outstanding unionist in Southland, he also played a valuable role in unifying the New Zealand labour movement.