McDonald said Sinn Féin will work collectively with other opposition parties who have already backed Connolly to give voters a “clear choice” and “hope”.
“We need a coalition that is about people, focused on making life better for workers, for families and communities, determined to create a future we can be proud of,” she said.
“This election provides an opportunity to bring together the widest possible coalition of voters to take on this failed and tired Government.”
The Sinn Féin leader said it is “wrong” that citizens in Northern Ireland cannot vote in the presidential election.
Only citizens living in the Republic of Ireland have the right to vote in Irish presidential elections under current rules.
McDonald said Sinn Féin activists and elected politicians have been consulted “on multiple occasions” and “had the opportunity to contribute” on who the party should back for the presidential election.
“We went about this in a very, very thorough way because we recognise that for a party that is a very large organisation with plenty of talent in our ranks, to support a candidate from outside of Sinn Féin, is a very big deal.”
She said Connolly’s commitment to the Irish language and history “all indicate her as a very, very powerful advocate of Irish reunification and of Ireland’s future”.