Cusack came to the wider public's attention when she starred as Dr. Kate Rowan in the popular 1960s television drama series Heartbeat (1992–1995). Her character tragically died from leukaemia in series 5 leaving her police man husband Nick, played by Nick Berry, a widower. Kate drove a cactus green Triumph Herald in the series. Cusack was nominated in the category of Best Actress in a TV Drama in 2004 at the Irish Television and Film awards IFTA for her role in the Cartlon Television TV film Too Good to be True. Niamh was nominated for a Whatsonstage.com Award in 2012 in the Best Supporting Actress in a Play category for her role in Playboy of the Western World at the Old Vic. In January 2013, she was nominated for a BBC Audio Drama Award in the Best Supporting Actress category for The Man with Wings by Rachel Joyce, produced by Gordon House, Goldhawk Essential Productions for Radio 4.
Cusack starred opposite Sean Bean in the 1986 RSC production of Romeo and Juliet. Throughout the 1990s Cusack worked regularly on the London stage in a series of leading roles including Nora Clitheroe in Sam Mendes's acclaimed production of The Plough and the Stars (Young Vic) opposite Judi Dench, Rosalind in As You Like It (Barbican), Flora in Tom Stoppard's Indian Ink (Aldwych) and The Maids (Donmar Warehouse). In the summer of 2003, she appeared as Portia in Shakespeare's The Merchant of Venice at the Chichester Festival Theatre, directed by Gale Edwards. In 2004, Cusack joined the Royal National Theatre in Nicholas Hynter's epic production of Philip Pullman's His Dark Materials as Serafina Pekkala alongside Anna Maxwell Martin, Ben Wishaw and Patricia Hodge. In 2007, Cusack returned to Royal National Theatre appearing in Victoria Benedictsson's The Enchantment, and played Alison Ellis in Crestfall by Mark O'Rowe at Theatre503. In 2009, she played Maggie in the first major revival in London of Brian Friel's multi award-winning Dancing at Lughnasa alongside her husband Finbar Lynch at the Old Vic Theatre. In 2010, she played Catherine Dickens in Andersen's English, a play by Sebastian Barry. In 2011 she appeared in The Painter by Rebecca Lenkiewicz, opposite Toby Jones and followed it with the role of Edith Davenport in Cause Célèbre by Terence Rattigan and The Widow Quin in The Playboy of the Western World by JM Synge, both at the Old Vic Theatre.
In 1989 Cusack took the part of an Actress, Valerie Saintclair, in the ninth episode of the first series of Agatha Christie's Poirot entitled The King of Clubs.
Other television acting credits also include Christine Fletcher in Always and Everyone (1999–2002), a British accident and emergency medical series alongside Martin Shaw; Grace Haslett in the miniseries State of Mind alongside Andrew Lincoln; Julie Flynn in the one off drama Rhinoceros alongside Robson Green and the small but important role of Beatrix Potter in the TV series The World of Peter Rabbit and Friends (1992–1995). She played Wodehouse's Bobbie Wickham in the ITV series Jeeves and Wooster (1990–1993). She played a character in an Agatha Christie's Marple series ("4:50 from Paddington", 2004), and has starred in episodes of Midsomer Murders (2008) and A Touch of Frost (2009) and the film The Closer You Get (2000), alongside Sean McGinle, for which she was nominated for an IFTA Award for Best Supporting Actor in a Film.
The production, which ran until late October 2012, was broadcast live to cinemas worldwide on 6 September 2012 through the National Theatre Live programme. The nominations for the 2013 Olivier Awards, which recognise excellence in professional productions staged in London, were announced on 26 March 2013; The Curious Case of the Dog in the Night-Time secured the most nominations with eight, including Best New Play, Best Director (Elliott), Best Actor (Treadaway), Best Actress in a Supporting Role, and other categories including Best Set Design, Best Lighting Design, Best Sound Design and Best Choreographer. The show transferred to the Apollo Theatre in Shaftesbury Avenue, London on 1 March 2013 with Cusack reprising her role of Siobhan. Following a period in film and television including the films Testament of Youth, Departure, Chick Lit and The Ghoul and the television series Rebellion, Cusack returned to the London stage as Paulina in The Winter's Tale at the Globe and Owen McCafferty's Unfaithful at Found 111 in the West End.
