UPDATE 2-Irish PM says Johnson call for new Brexit deal 'not in real world'

* Varadkar says EU's red lines have not changed

* Johnson's 'slogans and statements' need detail

* Says enthusiasm no substitute for policy (Adds more Varadkar quotes)

By Conor Humphries

DUBLIN, July 24 (Reuters) - Prime Minister Boris Johnson's suggestion that Britain's withdrawal agreement with the European Union can be completely renegotiated in the coming months is "not in the real world", Irish Prime Minister Leo Varadkar said on Wednesday.

The European Union's red lines had not changed and its negotiating position will not change ahead of Britain's planned exit from the European Union on Oct. 31, he added.

Varadkar was responding to a Johnson's vow in his first speech as prime minister to lead Britain out of the European Union on Oct. 31 "no ifs or buts" with "a new deal, a better deal."

"Listening to what he said today, I got the impression that he wasn't just talking about deleting the (Northern Ireland) backstop, he was talking about a whole new deal - a better deal for Britain," Varadkar said of Johnson's debut speech as prime minister. "That is not going to happen."

"Any suggestion that there can be a whole new deal negotiated in weeks or months is totally not in the real world," said Varadkar, who was speaking in an interview on RTE television.

Varadkar congratulated Johnson on his appointment and said he was looking forward to an "early engagement" on the Irish border issue, which is at the heart of the impasse in Brexit talks.

But he said his British counterpart would have to "put a little bit of detail behind some of those slogans and statements" about Brexit.

"Confidence and enthusiasm is not a substitute for a European policy or a foreign policy," he said. "We will need to hear in detail what he has in mind."

While Johnson said in his speech that he was willing to leave the EU without a deal if necessary, Varadkar said it was increasingly clear to him that Johnson did not have support in parliament for such a move.

He said he wanted ties with London to improve after they were damaged by Britain's decision to leave the European Union "without thinking it through or the impact on Britain itself, or on Northern Ireland or on wider relations with the EU." (Reporting by Conor Humphries, Editing by William Maclean and Jon Boyle)