Dónal Finn, the Cork actor who played Moriarty as a friend first

Dónal Finn

Photo: Hannahopstekop / CC0, via Wikimedia Commons

Born	August 31, 1995 Dromina, County Cork, Ireland
Occupation	Actor
Known for	Fantastic Beasts: The Secrets of Dumbledore, The Uprising, How to Build a Girl
Awards	Stalls to Stage · Sir Alec Guinness Award, Catholic Association of Performing Arts

Anyone who sat down to Guy Ritchie‘s Young Sherlock already knew how the story ends. James Moriarty is the Napoleon of crime, the man at the Reichenbach Falls, the enemy Sherlock Holmes was written to defeat. Dónal Finn plays him as something else: a quick, wry Irishman, a Kerry man by Finn’s own design, who becomes the young detective’s friend and whose grievances, the actor has said, are “justified because things have been hard for him.” Critics who came for Hero Fiennes Tiffin’s Holmes kept writing about the man standing next to him. For an actor who reached mainstream television by taking over a part someone else had already played, it was the first big role the audience did not already have a face for.

A farmhouse full of songs

The house he grew up in was crowded and loud. Finn was raised in Dromina, a village in north County Cork, one of eight children in a farming family where, as he has recalled, “as the night rolls on people do start to sing.” A school production of Aladdin sent him into the youth theatres around Mallow and Fermoy. In a local Little Shop of Horrors he played Seymour, and a regional reviewer singled out how he handled music considered very difficult to sing.

Drama school had to be paid for before it could be attended. After leaving Scoil Mhuire in Kanturk, Finn, born on 31 August 1995, spent a year working in a supermarket to cover audition and application fees. In 2015 the London Academy of Music and Dramatic Art gave him a three-year scholarship, and he left LAMDA in 2018 with a degree in acting and the Sir Alec Guinness Award from the Catholic Association of Performing Arts.

Small parts in big franchises

The first screen jobs were the standard apprenticeship of a young actor in British-made genre television. He was a peasant called Nettly in one episode of Netflix‘s The Witcher, a baker named Albert in Fantastic Beasts: The Secrets of Dumbledore, and one of the faces around Beanie Feldstein in How to Build a Girl. The stage gave him more room. He played opposite Daisy Edgar-Jones in Mike Bartlett’s Albion at the Almeida, appeared in Chasing Bono at Soho Theatre and crossed the Atlantic for the musical Sing Street at Boston’s Huntington Theatre.

Steven Knight’s SAS: Rogue Heroes then cast him as Eoin McGonigal, the young Irish officer among the founders of the Special Air Service, alongside Jack O’Connell and Connor Swindells. In the same year Amazon announced that he would take over Mat Cauthon in The Wheel of Time.

Inheriting Mat Cauthon

That casting is the knot at the centre of his career so far. Barney Harris had played Mat in the first season and left without any public explanation from Amazon or the showrunner, Rafe Judkins. Mat is one of the most loved characters in Robert Jordan’s novels, and Finn walked into a fandom that had already argued over one version of him. Copying Harris, he told TechRadar, would have made him “a shadow of what’s gone before”; he decided that “the best thing for myself and the fans was to interpret the role as my own.” The third season was widely judged the show’s strongest. Amazon cancelled it anyway in May 2025, citing cost and falling viewership. His Mat ended up as the fullest performance in a series that was never allowed to finish, more useful as a calling card for casting directors than as a role the wider public ever settled on. Not every bet paid off elsewhere either: Four Letters of Love, Polly Steele’s adaptation of Niall Williams’s novel, in which he played Sean Gore beside Pierce Brosnan and Helena Bonham Carter, drew mostly unfavourable reviews when it opened in Ireland and the UK in July 2025.

Orpheus, then Moriarty

The role that changed what people knew he could do was on stage. In February 2024 he opened as Orpheus in the original West End company of Hadestown at the Lyric Theatre, singing a part that leaves its actor nowhere to hide. He left the production in August, is heard on the cast album Hadestown: Live from London, and in 2025 won best actor in a musical at the first Stalls to Stage Awards. Screen International and Screen Ireland had already named him one of their inaugural Rising Stars of Ireland in 2023.

Then 2026 put him in two major series eleven days apart. Young Sherlock premiered on Prime Video on 4 March, and Amazon renewed it in April. On 15 March BBC One launched The Other Bennet Sister, Bad Wolf’s adaptation of Janice Hadlow’s novel, in which he plays Thomas Hayward opposite Ella Bruccoleri’s Mary Bennet; its first episode reached 7.3 million viewers. He followed both with a sold-out run as the teacher Carter Smith in the UK premiere of Kimberly Belflower’s John Proctor Is the Villain at the Royal Court.

Away from work he keeps a low profile. He lives in London and counts the Irish actors Chris Walley, Éanna Hardwicke and Frank Blake among his close friends.

A role nobody can rehearse

The Uprising, Paul Greengrass‘s film about the Peasants’ Revolt of 1381, with Andrew Garfield and Jamie Bell, reached American cinemas on 10 September and opens in the UK on 9 October; Finn plays a village priest. The second season of Young Sherlock is shooting in Sicily, and the show’s creator, Matthew Parkhill, has posted set photographs of Finn back in costume as Moriarty. On 1 November he takes his turn in Tim Crouch‘s An Oak Tree at The Other Palace in London, a play whose guest actor walks on stage without ever having read the script. For a performer whose career has been built on parts with a previous owner, it is an exact test: one night, one role, and nobody else’s version to measure it against.