Christian Eriksen's magnificent hat-trick inspired Denmark to a 5-1 demolition of the Republic of Ireland in Tuesday's second leg of their play-off in Dublin to clinch a spot at next year's World Cup.

Following Saturday’s stalemate in Copenhagen, Ireland got their home leg off to the perfect start when Shane Duffy headed them into the lead after only six minutes, but Denmark struck twice before the break and three more in the second half to seal their place in Russia.

"It is an incredible feeling. We have been fighting for so long to get to the World Cup. We are very much looking forward to it," said Eriksen.

"It is not often I score any hat-trick so of course it is incredible."

The equaliser arrived in fortuitous circumstances on 19 minutes when Cyrus Christie helped bundle in Andreas Christensen's effort, but there was nothing lucky about Eriksen’s thunderbolt for the second or his second-half brace.

Nicklas Bendtner, on as a late substitute, completed the rout in the final minute with a penalty after a clumsy trip on him by James McClean, extending Ireland's wait for a first World Cup appearance since 2002.

"We're really disappointed. We were well beaten in the end," said Ireland coach Martin O'Neill