He described it as a "pathetic sectarian state", during his speech to the annual meeting of the Ulster Unionist Council on Saturday.
The SDLP called for Mr Trimble to reconsider the comments, while Sinn Fein said Mr Trimble had "let himself down".
SDLP assembly member Alex Attwood said the comments said more about the UUP leader's attitude than they did about the Republic of Ireland.
"They were unfortunate, they were unneccessary and they were unhelpful," he said.
"You would like to think that they might be reconsidered."
Sinn Fein health minister Bairbre de Brun said the parties should be working together to move forward.
"I think a party leader's speech that is laced with one personalised attack after another simply shows that he should keep taking the tablets."
Included in Mr Trimble's speech was a call for a border-poll to be held in May next year.
He believes such a referendum, the first since 1973, would cement Northern Ireland's place within the United Kingdom.
But whether or not such a vote takes place is a decision for the Northern Ireland Secretary Dr John Reid.
On Saturday, a Northern Ireland Office spokesman said they had only just been made aware of the proposal, and a response would be made in due course.
The wording of the Good Friday Agreement only allows for a poll if it is thought there is a likelihood of majority support for a united Ireland.
Earlier on Saturday Mr Trimble was re-elected unopposed as party leader.
Census figures
If a referendum is to be held it will coincide with elections to the Northern Ireland Assembly which are due to take place in May 2003.
Should the Northern Ireland secretary approve a 2003 poll, the Agreement would only allow another poll after seven years, in 2010.
Many Sinn Fein politicians would be likely to oppose a poll despite the party's claims that last year's census figures, not yet published, show increasing numbers of Catholics in the province.
Sinn Fein chairman Mitchel McLaughlin welcomed Mr Trimble's call for a referendum
"We welcome his call for a border poll and look forward to discussing and debating with unionists how we can make a united Ireland acceptable to them."
Mr Trimble's call for a referendum was also welcomed by Mr Attwood.
A Downing Street spokesman said: "There are no plans for a referendum at this stage, but if Mr Trimble has proposals no doubt he will want to talk to us about them."
Speaking to the UUP meeting, Mr Trimble said a united Ireland would be disastrous and argued Northern Ireland currently offered "the right balance of Britishness and Irishness".