Fiona (33) appeared at the end of the first episode last night, playing the part of mysterious Jessica Hyde.
The Galway native will be making more show-stopping appearances.
Expect lots of high-energy chase scenes, explosions and even a torture sequence involving chilli powder, sand and bleach.
Utopia bills itself as a classic conspiracy series and has already drawn comparisons with hit Scandinavian drama The Killing.
O'Shaughnessy's character is a fighter on the run from a powerful organisation called The Network.
Irish actor Stephen Rea will also appear in the series.
Fiona won rave reviews for her role as Maggie in the Gate Theatre's sell-out production of Cat On A Hot Tin Roof in 2011.
She has since focused on breaking into film and TV.
Friends of the star, who used to date comic David McSavage, say 2013 is shaping up to be a big year for her.
"There is a big buzz about Utopia," a close friend said. "Fiona has got lots of offers of work in England and she wants to put all her energies into that."
Utopia was written by Irish writer Sharon Horgan's buddy, Dennis Kelly.
Funny woman Horgan and Kelly penned the critically acclaimed series Pulling, while Kelly also wrote the script for the West End musical Matilda.
The plot of Utopia centres around a group of graphic novel fans who meet up after becoming friends online.
"I didn't want to go down 'Geek Avenue' because lots of people read graphic novels," said Dennis. "You can reach a lot of ordinary people."
Utopia, Channel 4, Tuesdays, 10pm