Unite is calling for a public boycott of the Property Tax which, it said, could cause huge distress and despair in society.
The trade union, which is one of the country's largest, is urging the public to hold firm and start by not engaging with Revenue.
Unite has joined forces with the Campaign against Household and Water Taxes and will be participating in a day of protest on April 13.
The Revenue Commissioners have said they have sent more than a million letters to homeowners in recent weeks, and €1.3m has already been collected.
Regional Secretary of Unite Jimmy Kelly maintains they can still win the fight against the tax.
Mr Kelly said: "We want to build a campaign as strong as possible based on a boycott. The Government is just threatening people imposing a further austerity tax, calling it a property tax and threatening people by telling them they will take it out of their wages or social welfare or farm payments.
"We are saying 'Don't just go with those threats, you can resist this and we can have a victory on this campaign'.
"But it will take a long organising campaign to get to where it'll need to be."
