McLysaght is from Kill, County Kildare. After secondary school, she attended Trinity College Dublin, where she studied biology for a short time. In 2003, she switched courses to a Higher National Diploma in media studies at Ballyfermot College of Further Education where she met her friend and future co-author Sarah Breen. She graduated with a Bachelor of Arts in media production from Dublin City University, obtaining first class honours. In 2012, McLysaght completed a Master’s degree in international relations, again achieving first class honours. McLysaght dedicated her first book to her late father, who died in 2008.
McLysaght began her media career as a newsreader and reporter at Kfm radio station in 2004, moving to Newstalk radio for a brief period in 2006. Later in 2006, she moved to Phantom 105.2 where she was news editor for 4 years. She has also worked as a freelance journalist for the Business Post, the Leinster Leader, Radio Nova, and Today FM. In 2011, she joined TheJournal.ie before being promoted to deputy editor at TheDailyEdge.ie in 2013 and editor in 2017. In March 2017, McLysaght left her editorial role and became a full-time author. She is a regular columnist for The Irish Times.[non-primary source needed]
McLysaght and Breen co-created a comedic character named Aisling, establishing a Facebook fan group which grew to more than 45,000 members. As Aisling gained in popularity they were approached by publishers and decided to write a novel. As of 2021, the series consists of four books, with a further final book planned. The first three books were bestsellers and sold over 300,000 copies. They are published by Gill Books and Penguin Books.