Sinn Féin has stepped up its criticism of Ireland’s political establishment as polls show the party poised for a historic breakthrough in Saturday’s general election.

Mary Lou McDonald, Sinn Féin’s leader, said on Tuesday that Irish people wanted a change from Fine Gael and Fianna Fáil, centrist rivals that have dominated Irish politics for a century.

“People aren’t fools … There isn’t a whit of difference really between Fianna Fail and Fine Gael and people see that,” McDonald said before a television debate on Tuesday night with leaders of the three parties.

Earlier, an opinion poll gave Sinn Féin 25% support, vaulting it into top position for the first time and potentially heralding a realignment of Irish politics.

The party’s IRA links alienated voters during the Troubles in Northern Ireland but it is now appealing to people in the republic who chafe at homelessness, high rents, hospital waiting lists and other problems.

McDonald sought to fuel Sinn Féin’s momentum in Dublin – a key battleground - by assailing “ripoff” insurance companies and appearing alongside striking nurses on a picket line.

RTE’s televised debate lineup originally featured just Leo Varadkar, the taoiseach and Fine Gael leader, and Micheál Martin, the Fianna Fáil leader, but opinion poll results prompted the broadcaster to add McDonald.

The latest Irish Times poll showed Fianna Fáil dipping behind Sinn Féin to 23% and Fine Gael tumbling to 20%, with the Greens, small leftwing parties and independents making up the rest, casting uncertainty over which parties will form the next coalition government.

Fianna Fáil and Fine Gael have ruled out sharing power with Sinn Féin, saying it still has links to IRA figures, would ruin the economy and would be soft on crime, accusations the republican party rejects.

Fine Gael, fearing a wipeout after nine years in government, launched a counterattack on Tuesday. Simon Coveney, the foreign minister, accused Sinn Féin of fomenting “undercurrents of anger in society” and said its plan to increase public spending by €22bn (£19bn) in five years was reckless.

“The word ‘change’ has been ambushed and abused. Voters are being promised levels of spending and policies that would cripple the Irish economy again. Populism in the short term with a disregard for future consequence.”

Asked if Fine Gael would dump Varadkar if it haemorrhages seats, Coveney, a likely successor, said there was “no question mark” over Varadkar’s leadership.

Fine Gael had hoped its record on Brexit and the economy, which has bounced back from a 2009 crash, would clinch a third term. But voters have instead focused on the cost of living and fraying public services.

Fianna Fáil sought to ride the wave of desire for change but has struggled to distance itself from Fine Gael. The parties have similar policies and Fianna Fáil, citing Brexit threats, supported Varadkar’s minority administration in a confidence-and-supply deal.

John Toomey, 40, a factory worker in Listowel, County Kerry, said he was considering voting Sinn Féin for the first time out of frustration with the traditional dominant parties. “There needs to be a change. The health system is totally bonkers.”

Even Sinn Féin did not anticipate its surge. Chastened by losses in local and European elections last year it braced to lose some of its 23 seats in the 159-member Dáil assembly and fielded just 42 candidates, about half that of Fianna Fáil and Fine Gael.

This means an election surge will not fully translate into seats despite Ireland’s system of proportional representation. Fianna Fáil remains the bookies’ favourite to be the largest party and to lead the next government.

Northern Ireland has barely registered in the campaign but Sinn Féin has repeated its call for a unity referendum on both sides of the border by 2025.

The prospect of a Sinn Féin breakthrough south of the border – just weeks after it returned to power-sharing at Stormont – has jolted Northern Ireland unionists, said Duncan Morrow, a politics lecturer at Ulster University. “Unionists are rapidly trying to work out out what this could mean.”

In a surprise development Michelle O’Neill, Sinn Féin’s deputy leader and Northern Ireland’s deputy first minister, attended a Police Service of Northern Ireland recruitment event despite the party’s fractious relationship with policing. “I think it’s seismic and historic,” said the chief constable, Simon Byrne.