The guitarist died in April 2023 after a short illness at the age of 46.
Mark's death inspired the group's new single Both Ways, but Danny, who appeared as a coach on the first two series of The Voice UK, admitted it took several months for him to start writing songs again after his bandmate had lost.
The Script's upcoming album will give fans insight into their grief and mental health struggles following Mark's death.
Mark Sheehan (right) with The Script bandmates Glen Power and Danny O'Donoghue
Speaking to Magic Radio Breakfast presenters Ronan Keating and Harriet Scott, Danny said: “We were never going to replace Mark and, you know, achieve what that same thing was, so let's change the formation of the band.
“So now there are four of us and on the album itself you probably hear someone coming out of that depression.
“I still feel like I'm in the middle of it, so I still feel like I'm writing it as it's happening, so you know, there's a tribute to Mark on it.”
The 43-year-old opened up about his return to songwriting, revealing that he only picked up a pen at New Year's as he worked through his own personal struggles.
“I couldn't put it into words, I literally couldn't write anything for seven to eight months. You know, I started drinking pretty hard and really went off the rails,” he shared.
“Like you, I think, during those situations – and it would have been around Christmas. I'd had my fill of Guinness, you know, back home in Ireland, and I got back here on the 27th.
“So I came home to my house where I live in London and I just said, 'That's it, done.' I'm done drinking, I don't feel sorry for myself anymore'. It's not what he would have wanted.”
The band confirmed in a statement that Mark, who grew up in the Liberties in Dublin's south inner city, had died in a hospital in Britain on April 14 last year.
“Dearly loved husband, father, brother, bandmate and friend Mark Sheehan passed away today in hospital after a short illness,” they said.
“The family and the group ask that fans respect their privacy during this tragic time.”
Bandmate Glen Power later told the Sunday World how he and Danny were left “devastated” by the guitarist's shocking death.
“It is such a devastating loss for us, his family and everyone close to us,” he said last year.
“We had a connection that is very rare. We spent more time together than any couple. We are in shock, there are no words.”
The group formed in Dublin in 2001 and have sold more than 20 million albums, scored major hits and had major hits with the likes of The Man Who Can't be Moved, For The First Time, We Cry and Hall of Fame.