[15] Following advice given to him by Bret Hart,[14] he began training at Larry Sharpe's Monster Factory wrestling school in April 2002,[8] alongside Tank Toland and Cliff Compton.

After hitting an opponent in the groin, he gained the nickname of the "Irish Curse"[13] (a reference to a slang term supposedly denoting a man's inability to perform sexually after consuming an excessive amount of whiskey).

[14] Having attended an Irish-speaking school, he was aware of the four cycles of Irish mythology and, inspired by the artwork of Jim Fitzpatrick, incorporated sword- and axe-wielding imagery into his gimmick.

O'Shaunessy spent the rest of the year engaged in a rivalry with Vid Vain after losing to his tag team partner Joey Cabray the same night he won the battle royal.

[21] Despite these losses, his impressive showings earned him a place against Alex Shane in a guest match for the Frontier Wrestling Alliance (FWA) British Heavyweight Championship.

[23][24][25][26] This series of victories, aside from one loss by disqualification to Vain,[25] put him to a good form for the one-day tournament held in County Kildare to crown the inaugural IWW International Heavyweight Champion.

[34] After defending his title against Vampiro[35] and Viper,[36] O'Shaunessy found himself embroiled in a new feud with rising Scottish wrestling star "Thee" Drew Galloway, whom he later admitted was his toughest opponent.

In April 2007, he received another set of tryout matches in Milan and London against Galloway, as well as Sanders, WWE talent Jimmy Wang Yang, and Monster Factory alumnus Domino.

[8] O'Shaunessy debuted for the WWE developmental territory Florida Championship Wrestling (FCW) on 2 October in a double-debut match with a win over Bryan Kelly.

[63] By September, O'Shaunessy had focused his attention on singles competition and wrestled his way to the top of the card, successfully defeating former partner Hager for the Florida Heavyweight Championship.

[63] O'Shaunessy spent 2009 continuing to chase gold, earning two shots in January and February at the Florida Tag Team Champions Johnny Curtis and Tyler Reks, but he and Ryback were unsuccessful in both attempts.

He received a series of chances throughout the rest of the year, but could not win any more titles before being called up to WWE television,[64] unsuccessfully challenging Justin Angel for the Florida Heavyweight Championship in November.

[citation needed] 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 local competitor Oliver John.

[74] In the following weeks, he continued to attack Noble causing him to retire,[75] and, in lieu of competition, assaulted the timekeeper and commentator Jerry Lawler on 16 November.

[91] On 19 September at Night of Champions, Sheamus lost the WWE Championship to Orton in a six-pack elimination challenge, which also included Cena, Chris Jericho, Edge and Wade Barrett.

[93] Sheamus bounced back on 29 November, with wins over Kofi Kingston and John Morrison to become the 2010 King of the Ring, after receiving a bye through the second round.

He brought the United States Championship to SmackDown, but lost it back to the Raw brand at Extreme Rules on 1 May when Kingston defeated him in a Tables match.

[103][104] On the 3 June episode of SmackDown, Sheamus lost a World Heavyweight Championship match to Randy Orton due to biased refereeing from Christian.

[121][122][123][124][125] Sheamus went on to retain the title against Del Rio and Ziggler over the next few months, including at Money in the Bank on 15 July, SummerSlam on 19 August, and Night of Champions on 16 September.

[126][127][128][129] On 28 October at Hell in a Cell, Sheamus lost the World Heavyweight Championship to Big Show, ending his reign, which was the third longest in the title's history, at 210 days.

[139][140] On the 15 March episode of SmackDown, Sheamus and Orton were then allowed to pick a third partner to face the Shield in a six-man tag team match at WrestleMania 29 and chose Ryback.

[160] Sheamus returned to in-ring action on the 22 July episode of Raw, losing to World Heavyweight Champion Alberto Del Rio in a non-title match.

[161] On 7 August, WWE confirmed Sheamus had suffered a torn labrum in his shoulder that would require surgery, and was expected to miss between four and six months of ring action.

[166][167] On the 5 May episode of Raw, Sheamus won a 20-man battle royal to win the United States Championship for the second time, last eliminating previous champion Dean Ambrose.

[185] On the 30 November episode of Raw, Sheamus announced that he had formed his own stable called The League of Nations with Alberto Del Rio, Rusev and King Barrett, with the members of which having been born outside the U.S., while remaining associated with the Authority.

[216] On 29 January on the Royal Rumble kickoff show, Sheamus and Cesaro lost the Raw Tag Team Championship to Luke Gallows and Karl Anderson, and failed to regain them after The Hardy Boyz won the title at WrestleMania 33 on 2 April.

[223] The duo made their first televised defence of the title against The Hardy Boyz on the 12 June episode of Raw in a two out of three falls match where the two teams tied at 1–1 after being counted out.

[239] After going on a hiatus, the duo returned in late July to take part in a number one contendership tournament for the SmackDown Tag Team Championship and beat The Usos to advance to the finals,[240] where they lost to Big E and Kingston.

[289] On the 29 July episode of SmackDown, Sheamus faced Drew McIntyre in a "Good Old Fashioned Donnybrook" to determine the #1 contender for the Undisputed WWE Universal Championship at Clash at the Castle, but lost.

[310] In 2009, he appeared as a Celtic warrior zombie in the opening sequence of the film Assault of Darkness, and had a small part in Once Upon a Time in Dublin (aka 3 Crosses) as a security guard.