Damian Mackle is an Irish professional wrestler. He is best known for his time in WWE, where he performed under the ring name Killian Dain and since his departure from WWE he has reverted back to his ring name on the independent circuit Big Damo. That ring name Big Damo is derived from his first ring name, Damian O'Connor. Prior to joining WWE, he won the ICW World Heavyweight Championship while wrestling for Insane Championship Wrestling. He also wrestled for numerous promotions on the British independent circuit, around Europe, and in the USA including All Elite Wrestling, Progress Wrestling, Revolution Pro Wrestling, Westside Xtreme Wrestling, Absolute Intense Wrestling, Beyond Wrestling, Premier British Wrestling, Scottish Wrestling Alliance, What Culture Pro Wrestling and IPW:UK. On 19 October 2016, WWE announced that Mackle had signed a developmental contract with the company and had arrived at the WWE Performance Center in Orlando, Florida. He began to work in the NXT brand as part of the stable Sanity, along with Nikki Cross, Eric Young and Alexander Wolfe. In 2018, Dain, Wolfe and Young were drafted to SmackDown, where they worked as a stable until the dissolution. Then, he was drafted again to NXT, where he stayed until his release, in June 2021.