The Cold Feet star, who hails from Northern Ireland, returned to his homeland after a stint filming abroad and when he visited his mum on Mother's Day (14Mar10) he was shocked to see a marked deterioration in her condition, a form of incurable, degenerative dementia.
He says, "I hadn't seen her since Christmas as I've been filming a lot so for the first time in quite a long time I had not been able to get over. I noticed that when you spend a bit of time away the deterioration is, in a sense, more marked. She wasn't really aware of me when I visited her but by the end she knew me.
"On my last visit she did not know me but she knew dad. You just have to deal with it and we have responded as a family."
And Nesbitt admits that once he finishes his current acting job in Belgrade, Serbia he will be spending a lot more time in Northern Ireland with his mother.
He adds, "I hadn't been able to see her as often as I would have liked because my filming schedules have been so crazy and I was trying to get home to see my kids.
"When I finish the job in Belgrade I will come and spend a lot of time at home and I hope she will be able to remember better when she spends more time with me."