Leo Varadkar made the comment after Dominic Cummings suggested the UK had always intended to tear up the Brexit deal it signed with the EU in 2019.
The government said the deal had not worked as intended and must be changed.
And it accused the EU of failing to protect the Good Friday peace agreement in its implementation of the Northern Ireland Protocol.
During the campaign, he repeatedly claimed the withdrawal agreement he had negotiated with Brussels - including the Northern Ireland Protocol - was a "great" deal that was "oven ready".
He tweeted: "What I've said does NOT mean 'the PM was lying in General Election 2019', he never had a scoobydoo [a clue] what the deal he signed meant.
When the prime minister did finally comprehend, said Mr Cummings, "he was babbling 'I'd never have signed it if I'd understood it' (but that WAS a lie)".
Asked if Mr Cummings was correct in his assessment, Lord Frost said: "We all understood extremely well what the deal meant, it delivered on democracy, took the UK out of the EU whole and entire, and it was a very good deal.".
In a statement, the UK government said the protocol needed "significant change" to avoid further severe "economic, political and societal" disruption in Northern Ireland and to make it "sustainable for the future".
Mr Cummings - the former Vote Leave campaign chief - said that when Boris Johnson entered Downing Street in 2019, the country was facing the "worst constitutional crisis in a century" with much of what he called the "deep state" angling for "Brino" [Brexit in name only] or a second referendum.
Mr Varadkar told RTE television: "I hope Dominic Cummings is speaking for himself and not for the British government.
"Surely the message must go out to all countries around the world that this is a British government that doesn't necessarily keep its word and doesn't necessarily honour the agreements it makes.
The Taoiseach (Ireland's PM) Micheál Martin has given his backing to EU proposals for changes to the Northern Ireland Protocol, but he urged both sides to work "in good faith" and focus on "addressing disruption in trade between Northern Ireland and Great Britain".
Theresa May's former chief of staff, Lord Gavin Barwell, has, meanwhile, warned the UK's proposal for changing the Northern Ireland protocol has "no chance of success and is going to do even further damage to our relationship with our nearest neighbours".
He said he did not like the current Northern Ireland Protocol - but argued that the UK government had to meet the EU "half way".