The British government has sharply criticised Leo Varadkar's comments on Irish unity as "unhelpful and ill-advised".
Northern Secretary Brandon Lewis suggested Sinn Féin's recent surge in opinion polls "may explain the timing" of the Tánaiste's remarks.
He was speaking after Mr Varadkar told his party he believes a united Ireland can happen during his lifetime.
He also announced that Fine Gael intends to set up a political branch in Northern Ireland.
The Tánaiste told his party's Ard Fheis that Irish unity is a "legitimate political aspiration".
Addressing the House of Commons today, Mr Lewis said he was surprised at the comments from the Tánaiste.
He said: "We would be concerned about any deviation from the principle of consent, as enshrined in the Belfast/Good Friday agreement, but that agreement of course also respects the right of anyone to express their views, and we fully support that.
"We note the recent life and times survey, which showed support for a united Ireland at a low of 30% in Northern Ireland. I am also aware of the polls that put Sinn Féin ahead in the Republic, which may explain the timing of some of these comments from the Tanaiste.
"I urge everyone to dial down any rhetoric, particularly at this time of year, as it is unhelpful and ill-advised."
Mr Lewis was responding to a question from DUP MP Gavin Robinson, who called Mr Varadkar's comments "deeply unhelpful and destabilising".
The Tánaiste, meanwhile, has defended the timing of a speech saying Irish unification could happen in his lifetime.
He said: "I'd ask the counter-question - when is the good time?
"There'll always be a reason not to this week, next week, or next year... but my view in the round is that it's always a good time to talk about the future."
It comes amid high tensions in the North as parties try to re-establish the Stormont Executive following the DUP reshuffle and Arlene Foster's resignation.
There are questions about whether Sinn Féin will support the appointment of the DUP's Paul Givan as First Minister.
Issues around promised Irish language legislation - a key demand from Sinn Féin during the last Executive formation talks - continue to dominate negotiations in the North.