Joan was first diagnosed with MND last year and made the difficult decision to quit the Dublin soap before her condition deteriorates.
According to the actress: "I suppose it all started in October 2007. I was in the south of France with my neighbour and I was having a wonderful time; drinking wine in the evenings.
"One morning when we were standing on the platform in Cannes waiting for a train, I noticed I was slurring my words. We both joked that I was still drunk. But it persisted," she says.
Chat show host, Pat Kenny revealed he is delighted to have the privilege of honouring Joan's years as an actress.
"Brave Joan is joining us tonight to reflect on 20 years playing Mags Kelly and to tell us why raising awareness for Motor Neurone Disease is so important," he told the Herald.
Joan will also be joined by husband Willie Brosnan-Walsh and daughters Helen and Ingrid, not to mention a host of Fair City faces, all of whom are eager to pay tribute to a popular colleague.
Among those who'll be making an appearance are TV husband Tom Jordan who plays Charlie Kelly, Martina Stanley who is better known as Dolores Molloy, Jim Bartley who's worked with Joan from the very beginning as Bela and of course Dave Duffy who plays the character of Leo.
TV husband Charlie will reveal how he and Joan are the only couple in any soap that have stayed together for 20 years -- never having an affair or getting married again.
According to Tom: "I have lasted 20 years on the show because of Joan. She has a great sense of humour; we never had an argument or fell out. I'm a difficult person to get on with; so that is down to her. If anyone had any problems she would be the first to help. I am going to miss her terribly," he added.