The Sinn Féin leader says the wealth and prosperity generated by a united Ireland would far outweigh the cost.
Mary Lou McDonald's comments come after the Taoiseach said a Border poll would not be on the Government's agenda for the next five years.
Micheál Martin added that he would not say when, or if, a border poll would take place.
He made the comments at the launch of the Shared Island Initiative, which includes €500 million in ringfenced funding for joint North-South initiatives over the next five years.
Ms McDonald says a united Ireland would be beneficial on both sides of the Border.
"Some people focus on what this will cost, while others have done considerable work on the wealth and prosperity that a united Ireland would generate.
"The opportunity economically of Irish unification far outweighs anything that might be cited as regards the cost."
The €500 million funding will be spent on projects such as the A5 motorway to Derry, the Ulster Canal and the Narrow Water Bridge, according to The Irish Times.
In an interview with the paper, Mr Martin also said that he was of the view that "we learn to share this island" adding that Irish unity was a component rather than a priority.
The Taoiseach's views are not shared by many of his party members, with Jim O'Callaghan, Eamon Ó Cuív and Barry Cowen stressing that reunification should be at the core of the Shared Island unit, with Mr Cowen adding that a Border poll should be conducted by 2028.
