Arlene Foster has urged the Prime Minister to go back to Europe and seek a better deal.
She said the Irish Government should help find solutions to Brexit.
The DUP leader has also claimed there had never been a hard border on the island of Ireland.
She told Northern Ireland-based U105 radio: “For those of us who lived on the border and who were attacked by the IRA, we know that the IRA escaped across that border so it was not a hard border, nobody wants to go back to that.
“It takes the will to look for solutions and the regrettable thing is the Republic of Ireland has not been in the solution-finding mode.
“I hope that they are now, I hope that our Prime Minister uses that vote last night to go to Europe and to look for a better deal.”
The DUP’s 10 MPs voted against Theresa May’s draft EU Withdrawal Agreement on Tuesday.
Opposition had focused on the backstop, an insurance policy to avoid a hard Irish border after Brexit.