Now, as she takes her place as the oldest housewife on Wisteria Lane, the petite 52-year-old looks 15 years younger.
Attributing her youthful appearance to a relaxing-sounding mix of yoga, wine and sleep, she says that unlike her television neighbours, she has actually managed to gain weight since joining the show.
"When I started, the costume designer said, 'Don't get the Desperate Housewives disease', because everyone loses so much weight on the show. I think there is a competitive weight-loss thing. I failed completely. My costumes were too tight by the end of the season!''
Joining a well-established show during the fourth series is no easy task, so the writers needed a way to integrate Dana.
"They made it so that Katherine had lived on Wisteria Lane 12 years earlier and was an original housewife. That way she could come in and act like the alpha female, and treat the others like newcomers.
"I was a little taken a back at first by how mean she was,'' Dana admits. "But Marc, the writer, explained that he saw her as a JR-Ewing-from-Dallas type who goes in and takes control. As a result, people would come up to me in the street and say 'I hate you'.''
Playing an uptight woman who keeps her recipes and personal secrets locked away, Dana says she feels a lot of sympathy for Katherine, although they're nothing alike.
"She finds it hard to be vulnerable and once her secret is revealed you see why she is so damaged. By the end of the series, people were coming up to me and saying, 'I love you', you're my favourite housewife', and that's good. It means I'm doing my job.''
In the show, Katherine Mayfair makes an immediate "frenemy" in Bree, played by Marcia Cross.
"Marcia and I have a great time together,'' Dana says. "We were friends before the show and we just giggle. She makes faces that are so funny, it's very hard to keep a straight face.
Used to working on independent films and television projects, Dana says she has loved the chance to experience life on prime time television.
"I've never seen anything like the size of this production. Before I started the show I was doing an off-Broadway show by Neil Labute and was sharing a room the size of a table with six actresses."
There's also something contagious going around the set; Dana reveals that since joining the show, all of the other housewives have found love. "I said to the writer, 'I've noticed that all the women in the show seem to find a relationship --maybe it's me next'.
"I'm probably ready for marriage for the first time in my life. It has never been that important to me; I wanted to have adventures. I think that if I had married before, I would be divorced by now.'' She adds, giggling: "I'm off to Tuscany on holiday soon, so maybe there's an Italian waiter waiting for me.''
Desperate Housewives is on RTE 2 and returns to Channel Four on Wednesday, September 3