Mary Lou McDonald, who is seeking to become the country's first female prime minister, was speaking during a two-day visit to Lisbon by a Sinn Fein delegation. She has previously said she was sure a referendum would happen in the next decade.
"We are anxious to talk to people about this project and also about the EU itself and where it goes next," McDonald told reporters, adding she would "absolutely" travel to other European capitals ahead of the election.
Sinn Fein's leadership was to meet political leaders, including the Portuguese foreign minister, to discuss, among other issues, "the need for preparations to begin for Irish unity", the party said.
Under a 1998 peace deal, which largely ended three decades of violence between Catholic Irish nationalists and Protestant unionists, the British government can call a referendum on whether Northern Ireland should leave the United Kingdom if a "yes" majority looks likely.
Asked if she felt European leaders would support her party's quest, McDonald said people seemed "very engaged" and interested but that they needed to be sure a referendum "would not be chaotic" like the Brexit vote and subsequent negotiations.
Leaders abroad want to be confident that Sinn Fein would "do things in an orderly, thoughtful and democratic way" and Sinn Fein is happy to give that assurance, McDonald said.
Sinn Fein, the former political wing of the Irish Republican Army (IRA), is by far the most popular in Ireland ahead of elections due by early 2025 and became the first party seeking to leave the United Kingdom to come out on top at elections in Northern Ireland in May last year.
Sinn Fein's spokesperson on foreign affairs, Matt Carthy, travelled with McDonald to Lisbon and said the level of support for Ireland from European governments during Brexit negotiations was a shock to the British political system.
Their support, Carthy said, would also "be important in terms of ensuring that united Ireland will be a successful endeavour".
(Reporting by Catarina Demony; Additional reporting by Padraic Halpin and Conor Humphries; editing by Diane Craft)
By Catarina Demony
