Leo Varadkar has said removing the border backstop would be as bad for Ireland as a no-deal Brexit.
Some contenders to replace Theresa May as prime minister, including front runner Boris Johnson, have proposed changes to the policy.
The EU has said the Withdrawal Agreement – including the backstop – is not up for renegotiation.
The Irish premier told RTE’s Marian Finucane programme: “It’s a legal guarantee and legally operable guarantee that we will never see a hard border again.
“The difficulties we have with a time limit is effectively you are saying there will or could be a hard border once that time limit expires – that isn’t a backstop.
“What we are open to, and always have been open to, is alternative arrangements that perhaps could avoid a hard border, through procedures and technologies and so on.
“What we expect – and I don’t think it’s unreasonable – we want to see that fleshed out, we want to see it exist, it demonstrated before we are willing to give up the backstop.”
Mrs May’s erstwhile Democratic Unionist Party backers are adamant they cannot support any EU withdrawal deal which includes a backstop with no time limit.
The Taoiseach added: “What people are saying is ‘give up the backstop’ – which we know will work legally and operationally – in return for something that doesn’t yet exist but might exist in the future.
“I can’t do that to the border communities.”
Some Conservatives are concerned anything that keeps the UK tied to Europe after Brexit may hinder the ability to strike trade deals with other countries.
An extension to the period before the UK leaves the EU ends in the autumn.
Mr Johnson has claimed it is “perfectly realistic” to renegotiate the Withdrawal Agreement to allow the UK to leave in October.