Ennis grew up in Dublin, Ireland and studied at Maynooth University, graduating with a BSc in biology in 2002, and a PhD in ecology in 2008. After completing his doctorate, he worked as a biomedical scientist at ICON plc for a year before undertaking postdoctoral research at Concordia University in Montreal, Canada from 2010 until 2012. Since 2013, he has worked as a lab manager and research technician at the University of Oxford, where he specialises in researching the brains of insects.