Farrelly's inspiration into a professional wrestling career came from watching both British wrestling from ITV's World of Sport and American wrestling from the World Wrestling Federation (WWF) on Sky One. On the advice of Bret Hart, Farrelly began training in Larry Sharpe's Monster Factory wrestling school in April 2002, alongside Tank Toland, Cliff Compton, and Cindy Rogers. Six weeks later he officially debuted as a fan favorite under the name Sheamus O'Shaunessy against Robert Pigeon. It was during this time that he developed the nickname the "Irish Curse" after low-blowing an opponent. He soon suffered a serious neck injury after botching a hip toss which sidetracked his career for two years.
In May 2004, still using the ring name Sheamus O'Shaunessy, Farrelly returned to wrestling at the newly opened Irish Whip Wrestling (IWW) school in Dublin. He then made his debut match for the promotion at their Mount Temple show on 9 July against Mark Burns, where he picked out an easy win. He went on to win a battle royal the following month.
As well as making top contender appearances based on his growing reputation, O'Shaunessy also brought with him to Britain his long-standing rivalry with Galloway. He had already wrestled in Galloway's home promotion, British Championship Wrestling (BCW), twice in 2005, losing to Jay Phoenix on the day before Phoenix lost to Galloway in March, and returning in September to defeat one of BCW's top stars, Wolfgang, via countout. In the middle of their IWW feud the following year, O'Shaunessy went back to Scotland to take part in BCW's Path To Glory tour, defeating Galloway on the first night but conceding a loss to him the next night in May. Though their feud finished in Ireland in August 2006, it continued on across the Irish Sea when the two wrestled for the umbrella promotion Real Quality Wrestling (RQW) the following year. The two first met during April's Taking On The World show which ended in a double countout. The lack of a decision led to a rematch in June, and one of O'Shaunessy's last on the independent circuit, where Galloway picked up the victory in a No Holds Barred match ahead of an RQW Heavyweight Championship match.
Farrelly had a small role in the 2006 Irish film 3 Crosses, and played Two Ton in the 2008 movie The Escapist. He also appeared as a Celtic Warrior Zombie in the opening sequence of the 2009 film Assault of Darkness. He played Rocksteady in 2016's Teenage Mutant Ninja Turtles: Out of the Shadows.
On 22 July 2008, O'Shaunessy worked a dark match at a SmackDown taping, losing to R-Truth. The following year in May, O'Shaunessy began appearing at the Raw brand's house shows, and on both 8 and 9 May he defeated Jamie Noble along with another dark match victory over Noble before an episode of Raw.
On the 30 June 2009 episode of ECW, Farrelly made his unannounced debut as a villain under the shortened ring name of Sheamus, quickly defeating a local competitor. Sheamus soon entered into a critically well-received rivalry with Goldust after defeating him on 29 July. After exchanging victories in the following weeks they competed in a no disqualification match on 1 September which was won by Sheamus. Sheamus then began a feud with Shelton Benjamin which was hot-shot into a deciding match on 27 October which Sheamus won.
The following night on Raw, Sheamus won a "break-through" battle royal for Wrestlers who had never won a world championship, to become the number one contender to John Cena's WWE Championship. At the contract signing immediately afterward, Sheamus put Cena through the table, and the Raw guest host, Jesse Ventura, announced it would be a Tables match. On 13 December at TLC: Tables, Ladders and Chairs pay-per-view, Sheamus defeated Cena to win the WWE Championship, his first championship in WWE, making him the first Irish-born WWE Champion. He also captured the major title in just 166 days since his WWE debut, making it the third shortest time to capture a title since his WWE/ECW debut. The following night on Raw, Sheamus was awarded the 2009 Breakout Superstar of the Year Slammy Award. He would make his first title defense on the final Raw of 2009, getting disqualified in a rematch against Cena. On 31 January 2010 at the Royal Rumble, Sheamus defeated Randy Orton by disqualification to retain the WWE Championship, following interference from Orton's teammate Cody Rhodes. On 21 February, Sheamus lost the WWE Championship at the Elimination Chamber pay-per-view in the Elimination Chamber match after he was eliminated by Triple H. During the match, Sheamus suffered a concussion and as a result did not attend Raw the following night. Upon his return, he attacked Triple H in revenge, setting up a match at WrestleMania XXVI, where Sheamus lost. The following month at Extreme Rules, Sheamus attacked Triple H at the beginning of the show and later defeated him in a Street Fight. After a series of kicks to the head to win, he continued his assault after the match finished resulting in Triple H being written off television for ten months.
Sheamus made his video game debut in WWE SmackDown vs Raw 2011 and has since appeared as a playable character in WWE '12, WWE '13, WWE 2K14, WWE 2K15, WWE 2K16, WWE 2K17 and WWE 2K18.
On the 3 June episode of SmackDown, Sheamus lost a World Heavyweight Championship match to Randy Orton due to biased refereeing from Christian. Two weeks later, he lost a match to Christian, ending his chance to compete in the World Heavyweight Championship match at Capitol Punishment. Sheamus turned face for the first time, briefly feuding with Mark Henry over Henry being a bully, including a loss at SummerSlam, Sheamus went on a fourteen match winning streak that ended after interference from Christian, with whom he had been feuding. During this time, Sheamus defeated Christian in three consecutive matches at Hell in a Cell, Vengeance, and on SmackDown. He then went on a twelve match winning streak building towards winning the 2012 Royal Rumble. He entered the match twenty-second and won by last eliminating Chris Jericho to earn a main event championship match at WrestleMania XXVIII.
On the 7 November episode of Raw, Cesaro and Sheamus were announced as part of Team Raw for the 10–on–10 Survivor Series Tag Team Elimination match at Survivor Series on 20 November, which Team Raw won with Cesaro and Sheamus being the sole survivors of their team. The next night on Raw, Sheamus and Cesaro again failed to capture the tag titles from The New Day. After hinting at a break-up, the pair got into a bar brawl with men who offended them and finally began to cooperate with each other, turning Sheamus into basically a tweener (the fans still booed him while cheering Cesaro, but his previous heel viciousness had gone away). On 18 December at Roadblock: End of the Line, Sheamus and Cesaro defeated The New Day to win the Raw Tag Team Championship, marking Sheamus' first tag team championship in both his WWE and entire wrestling career. On 29 January on the Royal Rumble Kickoff show, Sheamus and Cesaro lost the Raw Tag Team Championship to Gallows and Anderson, and failed to regain them after the Hardy Boyz won the titles at Wrestlemania 33.
In July 2014, Farrelly was involved in the Royal Pains television series as a guest star.
On 19 July at the 2016 WWE draft, Sheamus was drafted to Raw. On the 25 July episode of Raw Sheamus was part of a fatal four-way match to determine the number one contender for the newly unveiled WWE Universal Championship, the match was later won by Roman Reigns. Sheamus would then begin a rivalry with Cesaro, after losing to Cesaro on the 1 August episode of Raw the two would continue to brawl after the match with match officials having to pull them apart. On the 8 August episode of Raw, after losing to Cesaro in a rematch, he would later cost Cesaro the United States Championship after interfering in his main event impromptu match against Rusev. The following week, Raw General Manager Mick Foley booked Sheamus and Cesaro in a Best of Seven Series, with the first match taking place on 21 August on the SummerSlam kick-off show, which Sheamus won. Sheamus would defeat Cesaro in the second and third matches, but lost to Cesaro in the fourth, fifth and sixth matches. At Clash of Champions, the match ended in a no contest, leaving their series tied at 3-3. The next night on Raw, Mick Foley decided to make the two partners to receive a Future WWE Raw Tag Team Championship opportunity. Later that night, they defeated a few local athletes, but still did not seem to be seeing eye to eye.
Cesaro and Sheamus won back the titles at the 2018 Royal Rumble, setting the record for the most reigns as a team at four. Earlier in the Royal Rumble match, Sheamus entered at number 11 and was quickly eliminated by Heath Slater, lasting just one second in the match. Following consecutive losses to Titus Worldwide (Titus O'Neil and Apollo) on Raw, Sheamus and Cesaro defended their titles against the two at Elimination Chamber, retaining successfully. After Braun Strowman single-handedly won a tag team battle royale to determine the number one contenders for the tag titles at WrestleMania 34 on the March 12th episode of Raw, Strowman stated he would reveal who his mystery partner would be at the event. At the event, Strowman revealed his partner to be a 10 year old fan called Nicholas. Nicholas and Strowman would defeat Sheamus and Cesaro, in turn making Nicholas the youngest champion of any kind in WWE history.
