It comes a week before a European Council summit at which EU leaders are set to "express concern" that "no substantial progress has yet been achieved" over the Irish border.
But finding a practical solution for customs checks needed for what will become an external border of the European Union after Brexit has so far proven elusive.
British Prime Minister Theresa May is struggling to overcome differences on Brexit within her Conservative Party and has clashed with parliament over what powers it should have over the final deal.
"A withdrawal agreement without a backstop is of no use whatsoever", he said.
He's being joined by the EU's Chief Brexit negotiator Michel Barnier.
Coming a week ahead of a key European Council meeting of leaders in Brussels, the visit to Dublin of Mr Juncker and the EU's chief Brexit negotiator Michel Barnier is a clear demonstration of the EU's solidarity with Ireland's position.
'We want to make it clear again and again that Ireland is not alone.
With just four months to go before the deadline for a Brexit deal, he warned: "The hardest parts are still to do and there is not much time left to find a concrete agreement".
SNP MPs staged a Commons walkout in protest over the way the EU (Withdrawal) Bill had been dealt with and Ms Sturgeon said Scotland had been "treated with utter contempt".
"Today's address not only afforded us the opportunity to hear directly from President Juncker, it also allowed for an honest and frank exchange of views across the parliament".
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"The UK gave the European Union back in December a guarantee there'd be no hard border on the island of Ireland".
Mr Juncker started his visit with a meeting with Taoiseach (Irish prime minister) Leo Varadkar and Irish Foreign Minister Simon Coveney at Government Buildings in Dublin.
The European Commission is now demanding the United Kingdom sign up to a Brexit "backstop" where Northern Ireland will stay inside the EU's Single Market and Customs Union, and be separated from the United Kingdom by a sea border until a way of keeping the Irish border open is found.
The majority of Brexit voters would still choose to leave the European Union even if it caused the United Kingdom to split, the latest Lord Ashcroft poll claims.
"Serious divergences remain, however, on the protocol on Ireland/Northern Ireland".
The Luxembourgian, 63, will receive an honorary doctorate from the National University of Ireland and attend an official dinner hosted by Varadkar in Dublin Castle later on Thursday.
"We'll be stepping up our preparations for all possible situations, and that does include a "no-deal" scenario".
He added: "Rather than focus on backstop arrangements, Ireland would be better served by a sensible new trade deal which would work for the European Union, the United Kingdom and, importantly, the Republic of Ireland".
But there will be no withdrawal agreement without a fully operation backstop.
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