The 33-year-old Ballance played 23 Tests and 16 ODIs for England between 2014 and 2017 but recently signed a two-year deal with the country of his birth.
The left-handed batter featured for Zimbabwe during the U19 World Cup before moving to England in 2006, where he initially represented county side Derbyshire before joining Yorkshire in 2008.
Ballance departed Yorkshire after admitting to using racist language against former team-mate Azeem Rafiq during a lengthy statement in late 2021.
Ballance is one of four changes made to the 15-man squad that represented Zimbabwe at the T20 World Cup in Australia in September-October.
Star all-rounder Sikandar Raza - who is in the running for the ICC Men's Cricketer of the Year, ICC Men's ODI Cricketer of the Year and ICC Men's T20I Cricketer of the Year awards - will not feature in the series against Ireland after being granted permission to play franchise cricket elsewhere.
Fast bowler Blessing Muzarabani, who is recovering from a quadriceps injury, has also missed out on selection alongside wicketkeeper-batter Regis Chakabva and batter Milton Shumba.
Apart from Ballance, the other new faces in the squad to be captained by Craig Ervine are Tadiwanashe Marumani, Innocent Kaia and Victor Nyauchi.
The T20I matches against Ireland will be played on 12, 14 and 15 January in Harare Sports Club which then be followed by three-match ODI series.
Zimbabwe T20 squad: Craig Ervine (captain), Gary Ballance, Ryan Burl, Tendai Chatara, Bradley Evans, Luke Jongwe, Innocent Kaia, Clive Madande, Wessley Madhevere, Tadiwanashe Marumani, Wellington Masakadza, Tony Munyonga, Richard Ngarava, Victor Nyauchi, Sean Williams