DUP MP Sammy Wilson has issued a warning after Sinn Fein's election success in the Republic of Ireland, saying they are "useless in government".
Mr Wilson served in the Stormont Executive alongside Sinn Fein as both Finance Minister and Environment Minister from 2008 to 2013.
The DUP's Brexit spokesperson also said that he believed if the party ended up part of a coalition government it could make the UK's Brexit negotiations easier as Sinn Fein's demands could be rejected out of hand by the UK Government.
His party colleague Sir Jeffrey Donaldson said Sinn Fein cannot afford to "mess up" the Stormont institutions after their stunning success at the polls.
Sir Jeffrey urged the party to use their success as a platform to "address concerns and enhance their democratic credentials as a political party".
The pair were speaking after Sinn Fein won the largest number of first preference votes in the Republic's General Election, ahead of the traditional big two parties, Fine Gael and Fianna Fail.
Sinn Fein have had conversations with other parties before they attempt to form a coalition government, but Mr Wilson said the party entering government was still far from certain.
Mr Wilson said that he believed having Sinn Fein in government would "probably make it easier" for the UK Government to reach a Brexit deal.
They are a foreign country, whatever they say about a UK referendum is immaterial Sammy Wilson
"I'm sure you'll get the usual anti-Brit stuff directed at the Government, but they already dealt with that from Fine Gael and Varadkar."
The South Antrim MP said that he believed any demands for a border poll would fall on deaf ears.
"They are a foreign country, whatever they say about a UK referendum is immaterial. Any demand from the Republic would be seen as an unacceptable intervention," he said.
"No amount of shouting can change the fact that there is no appetite for it.
"Boris Johnson has been very clear that he won't be giving Scotland a referendum so why would he give Northern Ireland one when that would just invite pressure from the Scottish?"
A triumphant Mary Lou McDonald during the election count
"I'm sure they will try and attempt a joined approach but while they trumpeted radical left-wing policies in the south up here they have to work within the constraints of a budget set by a Conservative Government and with their coalition partners.
"People down south will soon catch on, Sinn Fein are a good party of opposition where they can make all sorts of promises which bears no resemblance to reality, but they are a pretty useless party of government as we have learned over a number of years at Stormont."
We are very clear that there has to be political stability, that is what people want to see Sir Jeffrey Donaldson
Sir Jeffrey said that Sinn Fein still need to focus on maintaining good relations in Northern Ireland after their success.
"Sinn Fein will understand that if they mess up again in Northern Ireland it will have a negative impact on their support in the Republic," the Lagan Valley MP said.
"They would be foolish to seek the undermining the dealings of good government in Northern Ireland, the people they represent deserve it.
"We are very clear that there has to be political stability, that is what people want to see."
The DUP Westminster leader urged Sinn Fein to address concerns around the way the party operates.
"We would call on them to use this opportunity to to address these concerns and enhance their democratic credentials as a political party," he said.
Sir Jeffrey said that any talk around a Sinn Fein Foreign Minister was pre-mature given it was still unclear what form the new Irish government would take.
He said the issue would not concern his party as under the Good Friday Agreement the Irish Foreign Minister can not interfere in the domestic affairs of Northern Ireland.
Sir Jeffrey said that the party's electoral success should not be overstated.
"We respect their mandate, but the fact is that less than 25% of people in the Republic voted for them in this election and less than 25% of people in Northern Ireland voted for them in the last General Election," he said.
"Three-quarters of people in Ireland have not voted for Sinn Fein and we shouldn't lose sight of that."
A Sinn Fein spokesperson said: "Jeffrey Donaldson needs to read the latest in a series of polls which shows that more than 57% of people in the south support a referendum on Irish unity within five years."