Brennan was never actually declared in the Eastbourne Eagles team but made a number of guest appearances at reserve after he turned 15 as he was then eligible to ride in the National League. He made his National league debut for Eastbourne on his 15th birthday against Birmingham Brummies, however his debut didn't last very long as he crashed in his first ride and suffered a broken collarbone. Brennan returned from his injury and helped the Eagles win the National League Knockout Cup defeating Birmingham Brummies in the final.
2017 season
It was announced on 14 November 2016 that Brennan would be a member of the Eastbourne team for the upcoming season. Due to an incredible start to the season, after the first set of averages were released, Brennan moved from reserve straight to the top of the Eastbourne's averages meaning that he would ride at number 1 for the first time. He lost his place at number 1 to Georgie Wood at the end of June, and moved to number 4 where he rode for the majority of the rest of the season. He scored his first paid maximum scoring 9+3 from 4 rides in the National League Knockout Cup Final against Mildenhall Fen Tigers. Brennan finished the season with an average of 7.21, making him the most improved rider in the National League in 2017, with an increase of 4.21 from his starting average of 3.00. He also helped Eastbourne win the Knockout Cup for the third successive season, and the first time himself, beating Mildenhall Fen Tigers in the final.
2018 season
Brennan signed to ride for Eastbourne for a second season in the National League on 15 December 2017. He started in the number 3 position, where he got his first full maximum scoring 15 points from 5 rides on 14 April in a National League fixture against Buxton Hitmen, and then moved up to number 5. At the end of June, Brennan topped the Eagles averages and regained the number 1 race jacket. On 5 July, Brennan competed in the European U19 semi-final in Pardubice, Czech Republic. He scored 11 points from his five rides finishing 5th overall and therefore qualified for the Final. The final of the European U19s took place in Varkaus, Finland on 11 August. He finished 9th overall with a total of 7 points. Brennan made a very impressive SGB Premiership debut when guesting at reserve for Swindon Robins against Poole Pirates on 27 August, scoring a total of 3+1 from 3 rides, beating experienced Poole reserves Frederik Jakobsen and Kacper Woryna in the process. At the beginning of September he lost the number 1 jacket to Georgie Wood so returned to the number 5 position where he remained for the rest of the season. For the second consecutive season Brennan won the Knockout Cup with Eastbourne defeating Mildenhall Fen Tigers in the final. He also helped Eastbourne win their first league title in 18 years by winning the National League Play-Off Final against Mildenhall Fen Tigers.
2019 season
Eastbourne
Brennan joined Eastbourne again for a third season in a row, on 6 December 2018, continuing with them with their move up to the SGB Championship.
Cradley
With Eastbourne going up into the Championship, Brennan needed to find a new National League club. It was announced on 20 December 2018 that he had signed for the Cradley Heathens.
