Sinn Fein surged in Ireland's election. Here's why that's so controversial

The votes are still being counted but this left-wing, Irish nationalist party has pulled off a major political upset, breaking a century of dominance by establishment heavyweight parties (Fine Gael and Fianna Fail) and changing the political landscape of Ireland likely forever.

Here's what you need to know.

Does it mean Sinn Fein will be in power?

Sinn Fein won the most first-preference votes in Ireland's complex single-transferable-vote electoral system, but as they only fielded 42 candidates for 160 seats in the Dail (parliament), they are unlikely to be the largest party and therefore may not get to pick a new government or be invited to join one.

Why is Sinn Fein so controversial?

Sinn Fein appeared to have pulled off a major rebranding, seemingly burying their past as a party long accused of aligning with terrorism and violence.

Sinn Fein, although they repeatedly denied it, were the political wing of the IRA (the Irish Republican Army), who fought a bloody three-decade military campaign to throw the British out of Northern Ireland and unite the island of Ireland.

What were "The Troubles?"

The violence was known in an oddly understated way as "The Troubles," yet more than 3,500 people died and many more had their lives irrevocably changed.

The IRA was at the forefront of the conflict -- killing, bombing, shooting and intimidating their way to influence. They had grown out of a demand for equality in Northern Ireland's deeply bigoted society that often gave advantages to Protestants over Catholics.

As one of Sinn Fein's early politicians said -- they would rise through the Armalite [gun] and the ballot paper.

How has Sinn Fein rebranded itself and is it really a different party?

The moment Northern Ireland's Good Friday Agreement was signed in 1998, Sinn Fein President Gerry Adams pivoted south of the border rather than take a plum job in the north. He has always denied he was an IRA commander and saw that the political path to his years-long struggle for a united Ireland ran through Dublin. He has shaped the party accordingly, bringing in younger, less tainted politicians and leaders.

Two years ago, Adams stepped back as party president and did not run in this election after serving as a TD (or member of parliament) in Dublin for almost a decade.

The party surged through its grassroots activism around issues that captured voters' attention -- housing, homelessness and healthcare -- and their demands for change matured alongside a generation that never witnessed their violent roots. Even so, one their first elected TDs this weekend was a former IRA member, whose supporters sang rebel songs at the count center to celebrate his success.

Are Sinn Fein's gains likely to have an impact on Brexit talks?

Absolutely. Sinn Fein are now the only Irish party with major political influence both north and south of the Irish border, the European Union's new land border with the United Kingdom. Such is the seismic shift in Irish politics that Sinn Fein's demand for a united Ireland will be heard louder.

This will drive up growing Unionist fears in Northern Ireland, so whether he likes it or not, UK Prime Minister Boris Johnson's Brexit considerations will have to deal with this reality. Perhaps more directly, an Irish government with ardently pro-united Ireland Sinn Fein inside of it, or even in strong opposition to it, could stiffen the EU's resolve on negotiations and therefore limit potential concessions to the British.