The party has organised a number of public meetings on both sides of the Irish border to drum up support in its bid to be part of the next government in the Republic.
Mr Varadkar said the plans are an “unwelcome development”.
“Generally what happens in a democracy is people vote, the votes are counted and then parties try to form a government,” he said.
“What’s happening here ... it seems that Sinn Fein, having won less than a quarter of the vote, are behaving as if they have won a majority.
“My party regularly won more than a quarter of the vote and didn’t get into government.
“I think these rallies are designed to be the next phase in Sinn Fein’s campaign of intimidation and bullying.
“We saw that online and now we are seeing it in their rallies, and I wouldn’t be surprised if the next step is that they take it to the streets.
“It just shows you, again, that they are not a normal party; this is a party that has a casual relationship with democracy.”
Mr Varadkar is meeting Fianna Fail leader Micheal Martin today.
The Fine Gael leader previously said his party was preparing to go into opposition. Mr Varadkar denied that the meeting between the two leaders was a change in Fine Gael’s approach to government.