She studied for her teacher's diploma at the Guildhall School of Music and Drama, and spent ten years as a full-time actor.
[3][4] After leaving Fair City, Brennan completed an undergraduate degree in psychology at Maynooth University before pursuing a PhD on changes in the brain associated with aging with Ian Robertson at Trinity College Dublin.
[7] Through this work she has investigated the importance of building resilience for brain health through mental stimulation[8] and exercise[9] (amongst other factors), which has formed the foundations of much of her subsequent research.
She was co-director of the Neuro-Enhancement for Independent Lives (NEIL) research programme granted by Atlantic Philanthropies to develop interventions to slow or halt cognitive decline.
[17] In 2016, Brennan was a Trinity College Dublin candidate for a seat in Seanad Éireann but was unsuccessful.