The Ireland AM presenter managed to keep the split from husband Davy Ryan under wraps since September, she has said.
Sinead was not on the Ireland AM couch this morning -- but she was not expected to come in to work today.
The first any of her work colleagues knew about the break-up was yesterday, when TV3 boss Andrew Hanlon informed them of the news just as her agent Noel Kelly issued a formal statement.
Sinead and Davy married in a star-studded ceremony in France nearly four years ago, but separated last September.
A TV3 source said few of Sinead's work colleagues had known of her difficulties.
"The news of the split wasn't common knowledge around TV3. Most of Sinead's colleagues didn't even know until yesterday afternoon and were quite shocked and saddened to hear it.
"Sinead would be good pals with Mark (Cagney) and Alan (Hughes) but outside of that she would normally keep to herself and she wouldn't be the type to discuss her private life".
The insider confirmed that the break-up was not the subject of office gossip.
"Even when the news broke, people were quite respectful of Sinead's privacy and didn't really mention it.
"What happens outside of work is the individual's own business. Sinead has been working as normal on Ireland AM so she certainly hasn't brought it into the office with her."
Another colleague added: "We were completely in the dark. We had no idea what had happened."
Through her agent yesterday Sinead stressed that the couple were keen to deal with their break-up in private.
She said: "There is no one else involved. David is a wonderful guy and we are still great friends. This is a private and personal matter and we hope you respect our privacy at this time."
Sinead and Davy set up home in Glenageary, Co Dublin after their dream wedding in May 2009, when they married in front of over 100 family and friends in Aubeterre-sur-Dronne near Bordeaux. The pair met after Sinead came back to Ireland to take up the job with Ireland AM after a spell working as a features editor at a newspaper in London.
The TV presenter described her wedding as the biggest thing she had ever done and said at the time: "The idea of committing in front of everyone is nerve-wracking. I'm not good at finalising things.
"It took me four years to leave my last job and God knows how many years to leave my last relationship. I take this very seriously and things matter more to me know."
The previous year the bubbly TV host had been struck down with a suspected brain haemorrhage as she presented live on air. She made a full recovery.