Leo Varadkar has urged the UK Government to honour all the terms of the Withdrawal Agreement.
The Taoiseach was speaking after reports suggested that Prime Minister Boris Johnson ordered his Brexit team to find ways to "get around" the Northern Ireland protocol.
The Sunday Times reported that officials in Taskforce Europe are seeking to evade Irish Sea checks on goods passing from Great Britain to Northern Ireland.
Speaking in Dublin on Monday, Mr Varadkar said the Withdrawal Agreement is an international treaty and he expects the UK to "honour that in full".
The Taoiseach said there can be "no backsliding" and urged Mr Johnson's Government to focus on the next phase of negotiating a free trade agreement.
"We totally understand why they may want to minimise any checks at the ports and airports in Northern Ireland, we want to do that too, but the agreement clearly says if there have to be checks anywhere, they would happen at the ports and airports in Northern Ireland rather than on the land border between north and south," he said.
Mr Johnson's Cabinet will meet on Tuesday to sign off on the proposals, it was reported, which are expected to be presented in Parliament and published online on Thursday.
The Withdrawal Agreement between the UK and EU leaves Northern Ireland within the UK customs area but all EU procedures will apply to goods arriving there.
A UK Government spokesman responded to the reports, saying the UK will "comply with its obligations" under the deal struck with Brussels and said it would require only "limited changes" to Great Britain-Northern Ireland trade.
"The protocol specifically allows the UK to ensure unfettered market access for goods moving from Northern Ireland to GB," the Prime Minister's official spokesman said.
"The UK signed the Withdrawal Agreement including the protocol last month, we will comply with our obligations."
The spokesman added that Boris Johnson "has been clear that beyond the limited changes which are introduced by the protocol, there will be no changes to GB/NI trade".
"We have not asked any ports to prepare for new checks or controls between GB/NI."
Brussels said the Withdrawal Agreement must be effectively implemented and cautioned that respecting legal obligations would be "very important" for establishing trust between the UK and EU in future negotiations.
Dana Spinant, deputy chief spokeswoman of the European Commission, told reporters at a briefing: "The Withdrawal Agreement has been signed and ratified by both sides and as such it must be effectively implemented.
"That includes, of course, the protocol on Northern Ireland and all its constituent parts.
"Of course, in this context as in all others, we would expect our partners to respect their international obligations under their national law and under international law - that includes, of course, the Withdrawal Agreement which has legal force.
"Obviously, with respect to the future and how this plays in the negotiation, of course respecting our legal obligations is very important for establishing trust between two partners in a negotiation."