In 1995, she got her first TV role as Regina Crosbie in the serial Glenroe. She stayed for three years until 1998. She was offered her first feature film in 1997, All Souls' Day by Alan Gilsenan. She played a variety of roles in Irish films, including Drinking Crude (1997), co-starring Colin Farrell, and TV movie Miracle at Midnight (1998), with Mia Farrow. In 2002, Eva appeared in two dramas about Bloody Sunday: the documentary-style TV drama Bloody Sunday, starring James Nesbitt, and Sunday, written by Jimmy McGovern.
In 2003, she appeared in the TV series Trust before starring as Roisin Hanlon in the Ken Loach movie Ae Fond Kiss... (2003–04), which won her the 2005 London Critics Circle Film Award as "British Actress of the Year". She appeared in Breakfast on Pluto, Imagine Me & You and Save Angel Hope (by Lukas Erni) in 2005, and in Brian Kirk's Middletown in 2006. She starred as human rights Lawyer Jane Lavery in the TV conspiracy drama The State Within, and played Rosaleen in the Taken at the Flood episode of Agatha Christie's Poirot in 2006. In late 2007, she featured as Rembrandt's wife Saskia van Uylenburg in the historical drama, Nightwatching by Peter Greenaway.
She featured in the BBC drama The Last Enemy in early 2008, playing the role of Eleanor Brooke, a junior minister. In 2009, she portrayed Jenette in the last episode -season 2- of the BBC hit series Ashes to Ashes. She also appeared in two successful horror films The Children (2008) and Wake Wood (2011). In 2010 she won Best Actress at the Myrtle Beach International Film Festival for her acclaimed performance as a lovelorn paralegal in Curt Truninger's The Rendezvous.
She played "Annette Nicholls" in the 2010 three-part TV series Five Daughters. She appeared as Detective Superintendent Sarah Cavendish in the ninth, and final, series of Waking the Dead. In 2011, Birthistle appeared in the Sky1 TV series Strike Back: Project Dawn as Captain Kate Marshall. In 2013, Birthistle was co-starring Anna Friel in the Sky1 TV production The Psychopath Next Door. Since 2015, She has been starring in The Last Kingdom as Hild, a nun turned warrior and friend of Uhtred.
