In 2008, Niamh was featured in a show by photographer Simon Annand. The pictures included in the exhibition are of the 30 minutes before an actor steps on the stage in theatres. Niamh's picture shows her using the bathroom and her family made the request to Simon Annand for a print of it to give Niamh for her birthday.
In 1992, Niamh played Brigid Maguire in the film The Playboys alongside Albert Finney, Aiden Quinn and Adrian Dunbar.
In the summer of 1997, Niamh starred in a production of Jean Genet's The Maids at the Donmar Warehouse.
Niamh has performed in an audio book adaptation of Shakespeare's Comedy of Errors. Her sister Sorcha was also in the cast.
Niamh starred in The Three Sisters in 1990 and her partner, Finbar Lynch, played Tusenbach in the production.
In 2006, Niamh starred in Mammals, a comedy written by Amelia Bullmore and directed by Anna Mackim, at the Bush Theatre. The show received rave reviews and subsequently went on tour around the UK.
In 1991, Niamh was a guest speaker at the May Summer School for the Drama and Theatre Studies Department at Birkbeck, University of London.
In 1991, Niamh starred in a production of Sean O'Casey's play The Plough and The Stars alongside Dame Judi Dench at the Young Vic Theatre.
In 2004, Naimh was nominated for an Irish Film and Television Award in the category of Best Actress in a Drama for her performance as Tina in Too Good TO Be True.
Naimh is the voice of popular childrens' author Beatrix Potter in the animated series of the famous Potter books.
Naimh decided to leave the popular ITV 1960s police drama Heartbeat in 1995 when she became pregnant.
In 1990, she starred in a production of Chekhov's The Three Sisters at the Gate Theatre in Dublin alongside her real sisters Sinead and Sorcha and her father Cyril.
In 1986 Naimh played Juliet in a production of Shakespeare's Romeo and Juliet. Her two older sisters, Sinead and Sorcha, as well as her mother, Maureen Keily, have also played the part of Juliet in their acting careers.
In 2003 and 2004, Niamh played Serafina Pekkala at the National Theatre in a production of His Dark Materials, an adaptation of the Philip Pullman novels.
In 2003, Naimh took part in the Flora London Marathon in aid of St. Joseph's Hospice in Hackney in London.
When she was young, Naimh trained as a flautist at the Royal College of Music. She gave up music to go take up acting.
Naimh has performed at the Royal Shakespeare Company numerous times. She has played the following leading roles: Desdemona in Othello, Juliet in Romeo and Juliet and Rosalind in As You Like It.
In 1988 Niamh appeared on David Hare's Paris By Night alongside Charlotte Rampling and Michael Gambon.
She is the aunt of Samuel Irons.