Around noon on Easter Monday, when the Reflecting the Rising events had closed off the whole of O'Connell Street – Dublin's main thoroughfare - to make room for inspired costumed re-enactments and traditional music concerts, something completely unscripted happened.

Men and women in reflective sunglasses and military uniforms stepped forward. A pipe band struck up and flags were suddenly raised. They wore camouflaged scarves over their mouths and the hoisted yellow banners that read Republican Sinn Fein (RSF).

On the street people turned to look at each other. “Is this an official event?” I overheard someone ask incredulously. “Here come the shades!” said another, referring to their mirrored sunglasses. “They're just Sinn Fein wannabe's,” said someone else.

The group immediately began a fast march through Henry Street onto the wide avenue of O'Connell Street, where their pipe and drum tunes echoed the martial atmosphere of an Orange parade on the Twelfth of July.

The weekend's Rising commemorations had been remarkable for their dignified, progressive and at all times non-sectarian focus. Now came the forcible participation of a group of militant diehards who reject the Republic, reject the Peace Process, reject the politics of official Sinn Fein and believe they can win what wider Republicanism did not through four decades: the continuation of an armed struggle leading to a United Ireland, taken by force.

Irish police (Garda) instantly noted their presence but reacted slowly. Some officers forcibly removed the scarves the marchers wore to shield their mouths. But their parade still reached the GPO, the iconic center of the 1916 Rising, and a small time warp opened in the midst of 2016.

A speech was given by one of their number and concluded quickly. The wider public gave the event and its participants a wide berth, though. It was a reminder that for RSF, everything that has happened in Ireland since 1919 is illegitimate.

In the RSF worldview, independence has yet to be won. And in the economically deprived parts of the North where their numbers swell, that conclusion is underscored by their communal and economic isolation. A disaffected underclass in both the north and the south, they haven’t seen the point of either jurisdiction.

Above them on the Ionic columns of the GPO on Easter Monday the 100 year old bullet holes of the 1916 Rising were still visible. But sensing that they had been closed off from the sweeping narrative of official Ireland over the weekend, in both north and the south, they muscled in and sounded the only ominous note in a commemoration that was otherwise marked by solemnity and dignity.

For Dubliners it was a reminder that the challenges ahead for the next one hundred years still contain the most destabilizing seeds of the past one hundred.

We look away at our peril.