Much of her career has been in Irish and British-Irish productions, such as Michael Collins (1996), The Wind That Shakes the Barley (2006), Brideshead Revisited (2008) , Ondine (2009), The Guard (2011), Calvary (2014), The Rhythm Section (2020), Foundation (2021), and The Banshees of Inisherin (2022).
She has been nominated eleven times for Best Costume Design from the Irish Film & Television Academy, winning for The Rhythm Section.
In 2005, Ní Mhaoldomhnaigh was costume designer on the Neil Jordan film Breakfast on Pluto, which featured Cillian Murphy as a transgender woman.
Her designs included costumes made in Paris and London, featuring a strong colour palette and sensuous textures.
A reviewer in Irish Theatre Magazine praised Ní Mhaoldomhnaigh's efforts, writing that the "costumes are the real visual treat here, with the staid and stately conservatism of Civil War era dress recreated with great detail and grace".
[10] 2020 saw Eimer work with Reed Morano on the Barbara Broccoli-produced The Rhythm Section with Blake Lively and Jude Law.