IMPACT said the Government wanted "substantial" cuts in teachers' substitution and supervision pay, overtime and premium payments, and extra working hours, at talks on a deal.
It also told members "higher-paid" state employees would be asked to make a big contribution if there was to be an agreement, although the Government had not defined who they were.
The message, sent yesterday, noted that ministers were prepared to impose "at least" €1bn in payroll savings over the next three years if there was no deal, although they wanted an agreement.
"In a recent TV interview, the Taoiseach said the Government would legislate to secure at least €1bn, and elements in the majority government party would inevitably seek to extend the attack on our terms and conditions in the absence of a deal," said the union.
It described management's desire to conclude the talks by mid-February as "ambitious".
The union's warning of a Fine Gael threat comes after the Taoiseach faced the wrath of Labour backbenchers for discussing the possibility of more pay cuts while talks were under way.