The couple return to Weatherfield to announce their engagement in tonight's episode, but they will not remain together for long as Keith Duffy - who plays Ciaran - has already filmed the final scenes for his latest stint.
However, speaking on This Morning today, Marsh promised that the love between the pair will remain whatever happens.
"[Keith's departure] puts a question mark over what happens in their relationship," the actress explained. "But they are very much in love and even when Ciaran leaves, they're very much in love. So what actually happens leading up to that remains to be seen, but it is very good."
Marsh returned to filming at Coronation Street a few weeks ago as her maternity leave came to an end. She and fiancé Jamie Lomas welcomed the arrival of their baby daughter Polly in March.
Discussing her return to work, Marsh confessed: "It was really difficult. It was quite scary and the thought of not being with [Polly] at every second of every day was a bit harsh at first.
"When you have any time off and you come back, even after a holiday, you feel like the new girl again and it's like, 'I think I've forgotten how to do this!' So after ten months it was like, 'Oh no!' So I'm not expecting my first scenes to be very good!"
The 35-year-old also praised her comeback storyline, which sees Michelle support Carla (Alison King) amid her troubles with evil rapist Frank Foster (Andrew Lancel).
"It is brilliant, because I'm back in with a bang, really. It's good because I hit the ground running," she said. "The thing is that Carla and Michelle are sisters-in-law. As we know, Carla was married to Michelle's brother. They're very much a love-hate relationship, but it just shows that when the chips are down, they're there for each other. It's quite a shock, really."
Coronation Street airs Michelle and Ciaran's return tonight at 8.30pm on ITV1.
> Coronation Street Michelle 'to take on Frank Foster'
> Coronation Street star Kym Marsh teases Michelle, Frank battle
> Read more Coronation Street spoilers and news
Watch a video preview of tonight's Coronation Street episode below: