On September 1, 2015, it was reported that Lorcan had signed a developmental contract with WWE and would be reporting to NXT the following month. He made his debut at an NXT house show on October 30, in a Halloween battle royal, which was won by Bayley. On January 9, 2016, he made his singles match debut under his real name, losing to Rich Swann. At a set of television tapings held on January 22, he made his televised debut (which aired on February 24), in a losing effort against Apollo Crews. He then wrestled in a dark match before NXT TakeOver: Dallas, where he lost to Manny Andrade. On the June 22 episode of NXT, Lorcan defeated Tye Dillinger. On the November 15 episode of SmackDown, he made a main roster appearance, losing to Kalisto. On the December 14 episode of NXT, he lost in a match to Bobby Roode, losing the opportunity to be in a future fatal four-way match to decide a new number one contender to the NXT Championship. On April 4, he appeared on 205 Live, losing to Rich Swann.
From August 2017, Lorcan feuded with Danny Burch; after the two traded victories they formed a tag team and feuded with Riddick Moss and Tino Sabbatelli. In early 2018, they took part in the Dusty Rhodes Tag Team Classic, but were eliminated in the first round by team of Roderick Strong and Pete Dunne. Lorcan suffered a orbital bone fracture during his tag team match with Burch against The Undisputed Era at NXT TakeOver: Chicago, requiring facial surgery. Lorcan returned two months to the day of his injury on August 16, losing to War Raiders at an NXT live event. Lorcan spent the latter part of 2018 bouncing between the NXT and NXT UK brands. In February 2019, Lorcan and Burch were participants in the Dusty Rhodes Tag Team Classic tournament, but were eliminated in the first round by The Forgotten Sons.
In March 2019, Lorcan took part in a tournament on 205 Live to name a challenger for the WWE Cruiserweight Championship at WrestleMania 35. He defeated Humberto Carrillo in the first round, but lost to Cedric Alexander in the second round. On March 26, 2019, it was announced that he was moving to the 205 Live brand, but would still wrestle for NXT. He then redebuted on that nights episode of 205 Live, costing Alexander a match against Ariya Daivari and attacking him after the match, establishing himself as a heel in the process. On the April 16 episode of 205 Live, he defeated Alexander in Alexander's final match on 205 Live. After the match, Lorcan shook Alexander’s hand as a sign of respect, turning Lorcan face in the process. In August, Lorcan was declared the number one contender for Drew Gulak's Cruiserweight Championship, receiving his match at SummerSlam. At the event, Lorcan was defeated by Gulak. On the September 18 episode of NXT, Lorcan was defeated by Lio Rush in a number one contender's match for the Cruiserweight Championship.
On the February 13, 2020 episode of NXT UK, Lorcan teamed up with Danny Burch in a tag team match defeating The Hunt (Primate and Wild Boar). They then teamed up again on the February 13 episode of NXT UK in a non-title match against the NXT UK Tag Team Champions Gallus (Mark Coffey and Wolfgang) but failed to beat them. On the May 20 episode of NXT, Lorcan and Burch defeated Ever-Rise and proceeded to call out the NXT Tag Team Champions Imperium (Marcel Barthel and Fabian Aichner) afterwards by mocking their signature pose. The following week, they challenged them for the NXT Tag Team Titles.
On the October 21 episode of NXT, after Undisputed Era members Bobby Fish and Roderick Strong were taken out, Lorcan and Burch replaced them in an NXT Tag Team Championship match. They won the titles by defeating Breezango (Tyler Breeze and Fandango) with help from Pat McAfee, turning heel in the process. On March 23, 2021, William Regal vacated the NXT Tag Team Championship after Burch suffered a shoulder injury. On November 4, Lorcan was released from his WWE contract.