She succeeds Steffan Morris in the role, who left in June this year after playing with the group for five years.
Other members of the string group comprise violinists Johannes Marmen and Ricky Gore, and violist Bryony Gibson-Cornish. The group was formed in 2013 at London’s Royal College of Music.
O'Halloran is currently an Ambassador for the Benedetti Foundation and is co-founder and Artistic Director of the Ortús Chamber Music Festival. She plays on a Benjamin Banks cello c.1780, plus a baroque cello and bow on loan from the Royal Northern College of Music.
She will join the quartet in their debut at this year’s BBC Proms, and perform at the Three Choirs Festival in Worcester, Wonderfeel Helsinki Festival, and the Luberon String Quartet Festival.
The Marmen Quartet is an award winner at the 2019 Bordeaux and Banff International String Quartet Competitions. In 2018, they won first prize in the Final of the 2018 Royal Over-Seas League Annual Music Competition.
"I could not be more excited to share this incredible news with you!" said O'Halloran of her new appointment. "I am so delighted to be officially joining the amazing Marmen Quartet.
"It has been nothing but pure joy working with them over the past few weeks and I cannot wait for the amazing adventures that lie ahead for the four of us! We have so many exciting concerts coming in the next few months and I would love to see you there!"
"We couldn’t be happier to announce that Sinéad is joining the quartet," reads the group’s Facebook post.
"Her deep love for music was immediately apparent to us; she communicates with a rare generosity, warmth and imagination while sharing our endless curiosity towards the incredible body of repertoire we get to explore every day.
"Her presence in the quartet is an inspiration and the four of us are looking forward tremendously to connecting with you all on stages around the world!"