Marsh, who will play the legendary Elsie Tanner, said that her former co-star will join the Irish leg of the tour.
The 35-year-old told RTÉ's The Late Late Show: "At every show, we're going to have a special guest - at the end of the show - and they're going to sing along with the cast.
"We've just confirmed today that we have the lovely Keith Duffy joining us in Dublin and for Belfast."
Duffy's character Ciaran McCarthy had an on-screen relationship with Marsh's alter ego Michelle Connor in the ITV soap, until he set sail on a cruise ship in November last year.
On leaving the serial after a second spell, the Boyzone singer tipped producers to invite him back for a third stint.
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