The playmaker scored a sublime hat-trick as the Danes came from 1-0 down to triumph 5-1 in the second leg of their play-off at the Aviva Stadium. That was the aggregate score as well after the first leg ended 0-0 in Copenhagen.
Shane Duffy headed ROI in front on six minutes but an own goal levelled it on 29 minutes.
Christian then took over. He fired home off the underside of the crossbar from 20 yards for 2-1 on 32 minutes, curled a beauty from 22 yards with his left foot for 3-1 on 63 minutes and claimed the matchball with his hat-trick goal 10 minutes later, arrowing into the top corner from just inside the box. Nicklas Bendtner completed the scoring with a late penalty.
It was Christian's second international hat-trick and took his personal tally to 11 goals in an incredible qualifying campaign.
The youngest player at the 2010 World Cup in South Africa, aged just 18, Christian will now have a date in his diary - the draw for the finals is in Moscow on Friday 1 December.
Below: Christian curls home for his second and 3-1 on the night