[1] In 2016, Brennan made a number of guest appearances at reserve for Eastbourne Eagles after he turned 15 and was eligible to ride in the National League.
He made his debut for Eastbourne on his 15th birthday against Birmingham Brummies,[2] however he crashed in his first ride and suffered a broken collarbone.
[7] He also helped Eastbourne win the Knockout Cup for the third successive season, beating Mildenhall in the final.
[9][10][11] On 5 July, Brennan competed in the European U19 semi-final in Pardubice, scoring 11 points from his five rides finishing 5th and qualifying for the final.
[27] Also in 2023, he was part of the British team that won the silver medal in the 2023 Speedway World Cup final.