THE Northern Ireland Secretary has dismissed a call by Sinn Fein for a border poll to be held in Northern Ireland in the aftermath of the republican party's victory in the Stormont election.

Party president Mary Lou McDonald said on Friday five years was a “reasonable timescale” to prepare for a united Ireland vote and that preparations should start right away.

In an interview with TalkTV, Ms McDonald was asked if she would start to push for a reunification referendum.

"Well yes," she said.

"We believe that Irish unity is the best plan, the biggest opportunity for all of us who live on this island. Partition has been disastrous and led to conflict and no end of hardship. But it has to be done in a way that is planned, orderly, democratic and entirely peaceful."

She added: “In the first instance we need to start planning now for the change ahead. And that has to involve all of us and we believe it needs to be led by the government in Dublin in the first instance.

“I believe we are going to see these referendums — and there have to be two, north and south — in the coming years. Certainly within this decade we are going to see constitutional change on the island of Ireland.”

She added: "I believe that the referendum would be possible within a five-year timeframe.

"But much more importantly I believe that the preparation needs to start now.

"There will be no prize for anyone who buries their head in the sand or who will allow even the prospect of a disorderly reunification process. We saw Brexit disorder, lack of planning, lack of understanding, we are not going to repeat those mistakes on the island of Ireland."

In a speech yesterday, Ms McDonald told unionists “don’t be scared”, insisting the “future is bright for all of us”.

The Secretary of State has the power to call a border poll under the terms of the Good Friday Agreement which underpins the peace process in Northern Ireland.

The Agreement recognises the right of the people of the island of Ireland to bring about a united Ireland, subject to the consent of both parts. Therefore, in order for Irish reunification to take place, border polls must be held in Northern Ireland and the Republic of Ireland.

The Northern Act Act 1998 states that “if at any time it appears likely to him that a majority of those voting would express a wish that Northern Ireland should cease to be part of the United Kingdom and form part of a united Ireland”, the Secretary of State shall make an Order in Council enabling a border poll.

It is not clear exactly what would satisfy this requirement. According to the Institute for Government the Constitution Unit suggests that a consistent majority in opinion polls, a Catholic majority in a census, a nationalist majority in the Northern Ireland Assembly, or a vote by a majority in the Assembly could all be considered evidence of majority support for a united Ireland. However, the Secretary of State must ultimately decide whether the condition has been met.

Brandon Lewis was questioned on BBC Northern Ireland's Sunday Politics programme on the criteria for him calling a border poll.

The Northern Ireland Secretary responded by saying the nationalist vote has not gone up, and the unionist vote remains higher.

"Sinn Fein haven't gained seats, we haven't seen a growth in the nationalist vote and indeed the unionist vote is still larger and the number of seats held by unionist parties is still larger," he said.

Sinn Fein became the first nationalist or republican party to become the largest in the Northern Ireland Assembly with 27 seats, ahead of the DUP on 25.

Its northern leader and vice president Michelle O'Neill has said the party's victory in the Stormont contests "ushers in a new era" of politics.

But her hopes of becoming first minister in a powersharing executive in Belfast hinge on the unionist DUP, the second largest party, joining an administration - something it has ruled out unless there are major changes to Northern Ireland's post-Brexit deal.

Mr Lewis added: "I think the focus at the moment quite rightly is on getting Stormont back up and running, getting the money that is moving from the UK Government to Northern Ireland out to people in Northern Ireland so we can move forward on domestic issues, get the healthcare reform that people want to see and we want to get the protocol resolved for everyone in Northern Ireland."

He described the prospect of the first nationalist first minister as a "significant moment for Northern Ireland".

"I think it's an important moment to show that everyone can work together regardless of who is first and deputy first minister, that's what democracy is about and why it's important," he said.

Speaking to the BBC's Sophie Raworth deputy Prime Minister Dominic Raab was asked if we were seeing the beginning of the unravelling of the United Kingdom.

"I don't think so," he said.

"I think particularly after the elections in Northern Ireland there are two things that really do matter and one is to bring some stability to the situation. The people of Northern Ireland will want to see an executive up and running."

He was asked about Ms Donald's wish to have a border poll in five years.

"It was quite heavily caveated what she said," he said.

"But what I've noted is that 58 per cent of those voting in these elections voted for either those that support the Union or don't want to see constitutional change."

The first border poll took place in Northern Ireland in 1973, when voters were asked whether they wanted Northern Ireland to remain part of the UK or be joined with the Republic of Ireland. Some 99 per cent voted in favour of remaining in the UK. However, the poll was boycotted by most of the nationalist community; turnout was only 59 per cent.

The Good Friday Agreement states that consent for a united Ireland must be “freely and concurrently given” in both the North and the South of the island of Ireland. This is widely interpreted to mean that future border polls must be held in Northern Ireland and the Republic of Ireland at the same time.