[2] He competed in the men's light heavyweight event at the 2020 Summer Olympics.

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

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.