DUBLIN - 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 World Cup playoff in Dublin to clinch a spot in next year's finals.

Following Saturday's stalemate in Copenhagen, Ireland got its home leg off to the perfect start when Shane Duffy headed it into the lead after only six minutes, but Denmark struck twice before the break and three more in the second half to seal its 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 Tottenham star Eriksen.

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

The Danes equalized fortuitously in the 19th minute when Irish defender Cyrus Christie bundled in Andreas Christensen's effort, but there was nothing lucky about Eriksen's thunderbolt for the second or his second-half brace.

Former Arsenal striker Nicklas Bendtner, on as a late substitute, completed the rout in the final minute with a penalty after a clumsy trip 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.

"The two goals we conceded within a couple of minutes knocked us. We had to chase the game and Denmark took advantage. They deserved to go through."

O'Neill, who had already agreed a new contact to lead Ireland into the qualifiers for the 2020 European Championship, will be second-guessed for again selecting his team without the craft of wily veteran Wes Hoolahan.

The 35-year-old, one of four Ireland players likely to retire from international soccer, was only introduced at the interval with Ireland needing three goals to redeem its cause.

Duffy will certainly be around for the next campaign and his role as a central figure was solidified by his second crucial goal of the campaign.

After Daryl Murphy earned a freekick on the halfway line, Robbie Brady's delivery was telegraphed perfectly for the giant centerback to arrive on cue to nod past the onrushing Kasper Schmeichel.

That early breakthrough ensured the game opened up and Murphy nearly added a second only to ripple the side-netting with a volley.

Ireland now has a long wait until its next competitive matches - the start of the Euro 2020 qualifying campaign in March 2019.

"We've lost a playoff game and we'll come back again," O'Neill promised.

"This is hard work and we've only been beaten four times in 24 competitive matches since I came in," added the manager.

Former Aston Villa and Leicester O'Neill received plenty of flak for leaving Hoolahan out in order to put the returning David Meyler back into a midfield diamond that played to the strengths of Christian Eriksen.

"We were well beaten in the end. Although we got off to a great start by scoring first, a second goal for us might have made a difference," said O'Neill, who lamented the poor defending that enabled Denmark to take control of the second leg.

"We gave away two sloppy goals in the space of a few minutes and it was a long way back after that."

Agence France - presse