The Glasgow-born 23-year-old, who has played for Scotland at under-15s, under-19s and under-21s levels, has declared for Ireland after holding talks with manager Stephen Kenny, and the switch has now been approved by FIFA.
Johnston, currently on loan at Portuguese Primeira Liga club Vitoria Guimaraes, qualifies through his Derry-born grandfather and is now available to be included in the squad for the friendly clash with Latvia on March 22 and the opening Euro 2024 qualifier against France five days later.
He said: “I’m delighted to declare for the Republic of Ireland and excited for what my future holds in an Ireland shirt.
“Myself and my family have always been proud of our Irish roots with my grandad hailing from Derry and to potentially now play for Ireland is something that really excites me.
“It’s up to me now to impress the manager and to get selected to the squad, but my focus now is to break into that squad and to make an impact for the team in a big year for the team.
“I’ve always watched the Ireland matches and the atmosphere always looks incredible. I can’t wait to play at the Aviva Stadium and hopefully create some great moments for the team.”
Johnston made his Celtic debut as an 18-year-old under former boss Brendan Rodgers in 2017 and has gone on to win the Scottish Premiership three times, two Scottish Cups and two Scottish League Cups, although a serious knee injury cost him almost a year of his fledgling career and he has struggled to re-establish himself in Ange Postecoglou’s side.
The midfielder, who had rebuffed advances from Ireland earlier in his career, has also played Champions League and Europa League football and has made 20 appearances for Vitoria so far this season.
Kenny said: “Mikey is an exciting player, a great individual talent and we’re delighted to see him declare for the Republic of Ireland.
“I’m looking forward to seeing him play at the Aviva Stadium, dribbling in the way that he can and we look forward to helping him fulfil his potential in an Ireland shirt.”