Born in Lower Dublin Hill in Blackpool, a suburb of Cork, O'Farrell lived on Friars Road, in the Turners Cross area of the city.

In January 1948, after being spotted by West Ham scout Ben Ives, O'Farrell followed Moroney to the Upton Park club for a fee of £3,000.

[12] In November 1956, O'Farrell left for Preston North End in a straight swap deal involving Eddie Lewis.

He would then miss over a month of football after treatment for a nosebleed which caused him to lose around four pints of blood (he experienced a recurrence of the problem in the following season).

[24] He played infrequently over the next six years before making his ninth and last international appearance on 10 May 1959 in a 4–0 defeat by Czechoslovakia at Tehelne Pole Stadion in Bratislava.

[17] In 1961–62, he oversaw Weymouth's passage to the fourth round of the FA Cup, where they were beaten by his old club Preston North End.

[28] While Torquay manager he returned to West Ham to sign a number of players, including John Bond, Ken Brown and Bill Kitchener.

Leicester were near the bottom of the First Division table when he was appointed and were relegated at the end of the season, but O'Farrell led them to the 1969 FA Cup Final, which they lost 1–0 to Manchester City.

[35][36] O'Farrell's tenure started well, with Bobby Charlton, Denis Law and George Best playing well and the club, 10 points clear at one stage, losing just one of their first 14 league games.

[38] After being top of the league at Christmas, Best failed to attend training throughout January and United scored only three goals in their first eight games of the year, losing the first seven.

[40] O'Farrell signed Martin Buchan for a club-record fee of £125,000,[41] as well as Ian Storey-Moore, who scored in all of United's five league wins in the second half of the season.

[39] Forwards Wyn Davies and Ted MacDougall were signed in September 1972,[43][44] bringing O'Farrell's spending in the previous six months to £500,000.

[46] On 16 December, United lost 5–0 to Crystal Palace, with Don Rogers scoring twice,[47][48] and three days after the match, with the club third-from-bottom in the league, O'Farrell was sacked after 18 months in the role.

[51] His sacking resulted in O'Farrell suing the club over unpaid wages and he was forced to sign on at the local labour exchange while the dispute was settled.

[53] He began his tenure with seven consecutive wins, leading them to the gold medal at the 1974 Asian Games and qualification for the Montreal Olympics.

[31] He became Torquay manager again when Green left the club in May 1981, but once again relinquished the position the following year, this time in July 1982 on the appointment of Bruce Rioch.