During his time overseas he worked for Brian Dixon's All Star Wrestling, which gave him a taste of large audiences that he had not experienced in Ireland, as well as wrestling English veteran Robbie Brookside and other British wrestling luminaries, including Nigel McGuinness and Doug Williams. He also represented the UK in a Battle of the Nations tag team match between the UK and Austria alongside Drew McDonald and The Celtic Warrior in a losing effort against Chris Raaber, Michael Kovac & Robert Ray Kreuzer at the European Wrestlng Association's Night of Gladiators.
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 would lose 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 the to him 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.
On 13 November 2006, O'Shaunessy appeared on the World Wrestling Entertainment (WWE) show "Raw" at the Manchester Evening News Arena as part of the security team ejecting D-Generation X from ringside only to later receive a Pedigree from Triple H. The following day he and Galloway had a try out match against each other. The following year in April he received another set of try out matches in Milan and London against Galloway as well as Stu Sanders and WWE talent Jimmy Wang Yang and Monster Factory alumnus Domino. This led to an offer for signing a developmental contract for WWE from renowned trainer at which point he relocated to the United States.
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. While wrestling regularly, he was not part of any long feuds but did wrestle with a number of short-term tag teams alongside Hade Vansen and Jake Hager before teaming with Kafu (and managed by Dave Taylor) in the tournament to crown the inaugural FCW Florida Tag Team Champions. They advanced past the first round after defeating The British Lions ("Thee Superstar" Christopher Gray and "The Rascal" Tommy Taylor) but lost to Brad Allen and Nick Nemeth in the second round.
By September, O'Shaunessy had focused his attention to singles competition and wrestled his way to the top of the card, successfully defeating former partner Hager for the Florida Heavyweight Championship. He defended the title against Hager on 23 October and took on a number of competitors, with Eric Escobar and Joe Hennig consistently earning themselves into contention. Eventually, on 11 December, O'Shaunessy lost the title to Escobar in a four-way match also involving Hennig and Drew McIntyre (formerly Drew Galloway).
O'Shaunessy spent 2009 continuing to chase gold, earning two shots over January and February at the Florida Tag Team Champions Johnny Curtis and Tyler Reks but he and Ryback were unsuccessful in both attempts. In March he challenged the two for the titles once again, this time with McIntyre as his partner but could not defeat them, nor could he defeat McIntyre for the Florida Heavyweight Championship the following month. 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, unsuccessfully challenging Justin Angel for the Florida Heavyweight Championship in November.
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 the two appeared on the "Abraham Washington Show" talk segment leading to 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.
His rivalry with Benjamin ended prematurely because Sheamus had already been moved to the Raw brand. He made his "Raw" debut by defeating Jamie Noble on 26 October. In the following weeks, he continued to attack Noble causing him to retire, and, in lieu of competition, assaulted the timekeeper and commentator Jerry Lawler on November 16 at Madison Square Garden. The following week Sheamus appeared on his first WWE pay-per-view event, Survivor Series. Sheamus appeared as part of The Miz's team in a traditional five-on-five elimination tag team match. He eliminated fellow Irishman Finlay and made the final pinfall to eliminate the opposing captain John Morrison and survive along with Miz and long-time former rival Drew McIntyre.
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 afterwards, Sheamus put Cena through the table as the Raw guest host Jesse Ventura announced it would be a Table match. On 13 December, at the pay-per-view, Sheamus defeated Cena to win the WWE Championship, his first championship in WWE, making him the first Irish-born WWE Champion. The following night on "Raw", Sheamus was awarded the 2009 Breakout Superstar of the Year Slammy Award. On 31 January at the Royal Rumble, Sheamus retained the his title against Randy Orton via disqualification after he was attacked by 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 took revenge on Triple H with an attack to set up a match at WrestleMania XXVI that 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.
On 20 June, at the Fatal 4-Way pay-per-view, Sheamus won a fatal four-way match, following interference from The Nexus that allowed him to pin John Cena, thus winning the WWE Championship, and beginning his second World Championship reign. He went on to defeat Cena a third time in a steel cage match to retain the championship at the WWE Money in the Bank pay-per-view, and against Randy Orton at SummerSlam, which ended in disqualification, meaning that Sheamus retained his title. At Night of Champions, Sheamus was pinned by Randy Orton in a Six-Pack Elimination Challenge for the WWE Championship.