Mark Hamilton, an assistant chief constable with the Police Service of Northern Ireland (PSNI), confirmed that a 17-year-old girl, a 17-year-old boy and a 16-year-old boy had died in the incident, which took place at the Greenvale Hotel in Cookstown, County Tyrone.
"Our preliminary investigations show that there was a crush towards the front door of this hotel, and in that crush people seem to have fallen", Hamilton said in a press conference Monday.
The incident at the Greenvale Hotel on Drum Road happened at about 9.30pm.
"First and foremost my thoughts this morning are very much with the families of the three young people who passed away and those who were injured".
Speaking from the scene to the BBC, UUP councillor Trevor Wilson said: 'There's clearly a deep sense of shock here tonight that an event for young people just wanting to celebrate St Patrick's night went so horribly wrong'.
With only snippets of information to work off, it soon became apparent that two young people had died.
Mr Hamilton said officers were continuing to interview people who were at the event to "establish the full facts" of what happened.
Chief Inspector Graham Dodds said: "The St Patrick's event is now over, I would appeal to parents to contact their children, collect them immediately".
The ambulance service said: "NIAS despatched two rapid response paramedics, two doctors and five emergency crews to the scene".
Shortly after the incident police posted a message on Twitter, urging parents to pick up their children from the venue.
Northern Ireland Secretary Karen Bradley said: "I am deeply sorry to hear that three young people tragically lost their lives at a hotel in".
A police statement said: 'We can confirm there are two fatalities and two people are being treated in hospital.
"They say there was a huge crowd waiting to get into the venue and then a lot of commotion up near the front of the queue, then they said the emergency services arrived and they were all asked to leave".
"They were on their way home at that stage".
"My thoughts and prayers are with the families", she added.