Shane Duffy had headed Martin O'Neill's side into a sixth-minute lead at a sell-out Aviva Stadium but two goals in four first-half minutes prompted a decisive shift.

Eriksen struck just moments after Cyrus Christie's own goal (29) and then snuffed out home hopes with a fine second-half double (63, 73), before Nicklas Bendtner's 90th-minute penalty completed the rout.

Ireland had produced when it mattered in play-offs to reach their last World Cup finals in 2002 - as well as Euro 2016 - but though their start fleetingly stirred thoughts of another famous night, they were outclassed by a side for which Eriksen starred.

After toiling in Saturday's goalless first leg in Copenhagen, O'Neill had urged his side to seize the initiative at home and a boisterous crowd was bouncing when Duffy gambled on a Robbie Brady punt, punishing Nicolai Jorgensen's wayward clearance to nod over Kasper Schmeichel.

Shane Duffy put the Republic 1-0 up in Dublin

Denmark might have equalised just minutes later had Jens Stryger Larsen caught Simon Kjaer's swerving diagonal ball with boot rather than shin, while William Kvist went close with a low drive that Darren Randolph dived to stop with a firm hand.

A sweeping move down the left flank saw James McClean drag wide, the tempo from both sides brisk on a decisive night, but a moment of Irish misfortune and a flash of familiar quality put the Danes in the ascendancy.

Andreas Christensen's goalwards effort was turned over the line by Cyrus Christie

Chelsea's Andreas Christensen poked goalwards from a short corner, the ball bobbling off the goalframe and then back over the line off Christie, but Eriksen's follow-up (32) owed far less to luck. Stephen Ward carelessly surrendered possession on the halfway line but the break instigated by Yussuf Poulsen was swift and Eriksen struck without breaking stride.

O'Neill threw on Wes Hoolahan and Aiden McGeady at the interval but his side were sapped of confidence and vulnerable on the break.

Eriksen was set free with one counter, drilling straight at Randolph, but netted his second goal soon after, drifting into space before taking a touch and curling into the far corner.

He completed his hat-trick with a rasping drive into the roof of the net (73) as Ward faltered once again and there was still time for more Ireland misery, Bendtner punishing Duffy's foul from the spot as his side belatedly reached Russia in style.

Man of the match - Christian Eriksen

The night and the matchball belonged to Eriksen, who put the Danes ahead with his 32nd-minute thunderbolt and continued to inspire his country's comeback after the break. Shackled in Copenhagen, here he proved the difference, taking his tally for the campaign to 11 goals - just two fewer than the Republic managed in their 12 games in the competition. Tottenham fans will hope for more of the same in Saturday's north London derby, live on Sky Sports Premier League.

What's next?

While the Republic will reflect on a further stretch in the World Cup wilderness, Denmark can look forward to the draw for next summer's tournament on December 1.