Mark Chamberlain knocked out Pierce O'Leary in the tenth round to win the IBO light-welterweight title at the 3Arena in Dublin on 1 August 2026. The finish came in front of a capacity crowd of around 7,000 — the Dubliner's first professional fight on home soil. BBC Sport

O'Leary entered the ring as the defending champion, unbeaten in 19 professional fights with 11 knockouts. He had claimed the vacant IBO super-lightweight belt — the same 140-pound division, known interchangeably as light-welterweight — by beating Maxi Hughes earlier in 2026. Queensberry Promotions World Boxing News

For Chamberlain, the assignment was his second at super lightweight. He arrived with a record of 17 wins and one defeat, his only loss coming against Josh Padley in 2024. The O'Leary bout was his first fight since drawing with Jack Rafferty in Altrincham the previous summer — a long layoff by any standard. BBC Sport

The fight turned dramatically in the seventh round. O'Leary landed a clean body shot that put Chamberlain on the canvas. Chamberlain beat the count, survived the round, and kept coming.

By the tenth, the momentum had swung entirely. Chamberlain found the finish, stopping O'Leary and silencing the home crowd. The win takes his professional record to 18 wins and one defeat, and makes him the new IBO light-welterweight champion. BBC Sport

The IBO is one of several bodies that sanction world title fights in boxing, sitting alongside the more widely recognised WBA, WBC, IBF and WBO. A belt from any of them carries world-title status; the IBO is often a stepping stone toward those larger organisations.

The bout was billed by Queensberry Promotions as co-main event of the card at the 3Arena. Queensberry Promotions

For O'Leary, the defeat is the first blemish on a professional résumé that had been building steadily since his amateur days, when he posted 56 wins against 7 losses with 15 knockouts. He had defended the IBO belt he won from Hughes — a seasoned operator with a 29-9-2 record — and carried genuine momentum into the Dublin homecoming. BoxRec World Boxing News

For Chamberlain, the night is the clearest sign yet that his move up to super lightweight is paying off. He recovered from a genuine crisis — knocked down, hurt, fighting in the opponent's backyard — and found a way to finish. That is the kind of win that shifts a career from prospect to contender.