[4] In the Irish National Boxing Championships Brennan won silver in 2019, sandwiched by gold medals in 2017 and 2020.

In the Olympics, Brennan was eliminated from the tournament in the Round of 32 by Dilshodbek Ruzmetov.

Following the 2020 Summer Olympics, Brennan developed a drinking problem due to difficulties in recovering from a shoulder surgery.

However, he eventually sobered up, got into contact with Darren Barker and embarked on a career in professional boxing.

[8][9] As of October 2024, Brennan had racked a record of 4–0 in his tenure in professional boxing.