Today she's buzzing with excitement not only about Shakespeare's comedy, with its mixed-up plot of cross-dressed twins and mistaken identity, but about doing it in Scotland with Scottish actors. "Working in Ireland and being Irish, you're aware there's a Celtic sensibility that is different from an English one. I'm not saying Ireland and Scotland are the same, but there is a cultural inheritance that is shared."
Determined to engage with her new home, she and her creative team have been venturing into the Perthshire countryside to find inspiration for the setting of the play, which she is updating to the 1920s. She's not making an explicit connection, but in her head, the play's Orsino could be the Duke of Atholl, his home might be Scone Palace and Perthshire could be Illyria. To maintain the geographical consistency, she's given the roles of the shipwrecked twins, washed up in an unfamiliar Scotland, to Irish actors.
"It's loose, but I wanted to get a sense of the country I'm directing in." O'Riordan, is working with long-time collaborator Conor Mitchell, the composer behind the Fringe First-winning Ten Plagues with Marc Almond and Tuesdays at Tescos with Simon Callow.
"I've been going around on the train a lot with the set designer and lighting designer and just stopping off and spending afternoons in bits of the lower Highlands. It's so mind-blowingly beautiful. The quality of light is particularly stunning and I'm trying to get a sense of space and light in this production."
Having had a masterclass in verse speaking when she worked under Sir Peter Hall on Measure For Measure, she is delighted by the way her actors are handling the text: "The way the verse and prose is spoken in a Scottish accent is very exciting because the last thing Shakespeare's actors would have spoken in is an RP accent. There's such a muscularity and clarity in the Scottish accent, really embracing the consonants, finishing words off, and it just brings the text vividly to life."
O'Riordan switched to directing after training at the Royal Ballet School and the Kirov Ballet and spent her twenties as a choreographer and movement director, but she made her name in 2002 when she turned director for Hurricane, a play about snooker legend Alex "Hurricane" Higgins, starring her husband Richard Dormer.
Transferring from Belfast to Edinburgh's Assembly Rooms and on to London, it took her "from 0 to 60" and kick-started a career that has included stints in Bath, London and Manchester. Seeing Perth Theatre for the first time at her interview, she knew instantly this was the job for her. "I walked into that auditorium and just went, 'Jesus!' because it's beautiful."
Cheered by the reception of The Absence of Women, a play she directed for Belfast's Lyric Theatre and which did a short run at Perth earlier this month, she wants to entertain the theatre's existing loyal audience while broadening its appeal with new strands of programming. In February, she'll be directing Someone Who'll Watch Over Me, Frank McGuinness's powerful play inspired by hostages Brian Keenan and John McCarthy, followed in March by the Scottish premiere of Moonlight And Magnolias, a comedy about the behind-the-scenes machinations on the set of Gone With The Wind. That production will also play at Glasgow's Tron.
Additionally, she'll be branching out with a visit from Grid Iron's Fringe First-winning Barflies, performed in the bar, and a commission for Perth playwright Ben Tagoe on a co-production with Glasgow's A Play, A Pie And A Pint.
"The audience here is an educated audience that knows what it likes," says O'Riordan, who is planning co-productions with other Scottish and Irish theatres, toying with the idea of a site-specific production of her own, and is excited by a plan to give the theatre a more welcoming glass frontage.
"I'm not here to change our audience, I'm here to add to our audience. My job is to lead the audience, to be their friend, it's not to bully them or say, 'I know better than you.'"
Until Orla O'Loughlin arrives at Edinburgh's Traverse Theatre in January, she is the only woman in charge of a building-based theatre company in Scotland, though she says that as a director she has tastes that could be described as masculine. Perhaps that's because she has three younger brothers or maybe it's simply because she believes theatre should have a sense of urgency.
"What I like on stage is a tough quality. I like brave choices. I like the audience to feel something is happening that needs to be on a stage, that couldn't happen on a telly screen or radio. That means a physical understanding of the text and a robustness of how you convey it – and it needs to be entertainment."
• Twelfth Night is at Perth Theatre until 15 October. www.horsecross.co.uk