"[1] McElwain completed her Bachelor of Arts degree in Botany from Trinity College Dublin (TCD) in 1993 and her PhD in Paleobotany in 1997 from the Royal Holloway, University of London.

[2] During her first year at TCD, she was influenced to pursue a career as a palaeobotanist after enrolling in a course focusing on quaternary palynology and geomorphology of the Irish landscape.

[2] As a postgraduate student at Sheffield University, McElwain studied the impact carbon dioxide had on global warming by examining plant fossils collected in Greenland during the 1920s.

[8][9] Two years later, McElwain accepted a faculty position at University College Dublin[10] and received the Award for Excellence in EU research by the President of Ireland in 2012.

[2] McElwain continued her research into CO2 in the atmosphere, and led a study in 2019 which found that holly and ivy are more climate change-ready in the face of warming temperatures than birch and oak.