McGrath, born January 3, 1983,[3] was raised inAshford,County Wicklow, by her father, Paul, who works with computers, and mother, Mary, who works for an Irish designer. She has two older brothers, Sean, an online media manager, and Rory, who is a post-production producer.[4] She studied theInternational Baccalaureate atSt. Andrew's College before graduating fromTrinity College, Dublin, with a degree in history with a focus on Russian history.[1]
At a young age she applied to the Ballygowan "Be the Face of MTV" competition.[5] Twenty finalists would be able to attend MTV Lick Party at the Tivoli.
McGrath originally sought to work in fashion journalism and took on a job atImage magazine[1] before becoming a wardrobe assistant on the set ofThe Tudors. While working onThe Tudors, McGrath was advised to try acting, so she sent photographs to Irish agents.[6] She said:
Being an actress was kind of like when you're a kid, and you want to run away and join the circus, it's something you really want to do, but then you grew up and got a proper job. It was a dream, but I didn't think it would ever be reality, and yet here I am![6]
McGrath was cast inDamage, an Irish TV-movie in 2007. She also starred in the playLa Marea at theDublin Theatre Festival in the same year.[7] She appeared in the feature filmsEden andFreakdog in 2008, before being cast as Morgana Pendragon inMerlin.[8]
In 2011, McGrath appeared withRoger Moore in the comedy-drama filmA Princess for Christmas, aHallmark Channel Original Movie shot mainly at thePeleş Castle in Romania.[11] In September 2011, McGrath voice-acted the characters in the Irish animated short filmTríd an Stoirm (Through the Storm).[12] Later that month, McGrath was cast as Oriane Congost inLabyrinth.[13]
McGrath was reunited with herThe Tudors co-star and close friendJonathan Rhys Meyers inNBC andSky Living's horror drama TV seriesDracula; she portrayedLucy Westenra.[14] In June 2013, McGrath co-starred in episode four of the Channel 4 showDates as a young lesbian on the dating scene alongsideGemma Chan.[15]
In November 2014, McGrath co-starred in aHozier music video for the song "From Eden".[16]
McGrath portrayed the lead role in the first season of Chiller's original horror seriesSlasher, which premiered on 4 March 2016.[19]
In 2016, it was announced that McGrath would play the recurring role ofLena Luthor in the second season ofSupergirl.[20][21] She debuted in the season two premiere episode titled "The Adventures of Supergirl" and was promoted to a series regular in March 2017 and maintained this role until the show's end in 2021.[22][23] She appeared as Elsa inGuy Ritchie's fantasy filmKing Arthur: Legend of the Sword, which was released in May 2017.[24]
In 2019, McGrath starred alongsideAbbie Cornish andGeorgina Haig inSeven Network seriesSecret Bridesmaids' Business, about how a bride's wedding turns deadly after one of the bridesmaids invites a stranger into their lives. McGrath starred as Saskia, a bisexual lawyer who is described as an intelligent, loyal woman who is holding a secret.[25]
In September 2023, she had a supporting role inThe Continental as the Adjudicator.[26]