In a statement, Irish Rail said the 1.45pm Connolly/Bray train was forced to stop on its approach to Bray.
The railway company has now suspended services through Bray and apologised to customers for the “delay and upset caused”.
“A southbound DART was awaiting a clear platform at Bray before proceeding into the station, when a number of people forced open doors and walked on the track,” the statement said.
“This dangerous act means that trains have to be stopped in both directions while our staff and gardaí clear trespassers from the line, resulting in delays for customers on board this and trains behind it, and a similar issue has occurred on the DART behind.
“While people are understandably anxious to reach the station, this has turned a delay outside Bray of just over 5 minutes into one which may impact services for up to an hour.
"Staff are at stations and on board updating customers, as we work with gardaí to clear the line.”
In relation to issues with air conditioning on board, Irish Rail said it has “no reports” of any such issue.
"Our DARTS either have aircon or openable windows, and we have no reports of issues with either," it said.
"A very busy train stopped on the line on a warm day can feel warm regardless, but again, forcing open doors only means that if any customer was experiencing issues, it delayed any assistance, as there are extensive ambulance and medical personnel in the Bray Station area as part of arrangements for the Air Show.”
In a statement issued to Independent.ie, a garda spokesperson said: “Gardaí and emergency services currently at the scene of an incident on rail line near Bray train station.”
“There is currently a disruption to rail services in the area. Gardaí are assisting with traffic management. No further details are available at this time.”
In a post on Twitter today, the railway company said: “13:45 Connolly/Bray stopped approaching Bray due to passengers forcing doors open and trespassing on the line. All services into Bray are currently stopped as a result.”
“Control have alerted drivers of aircon issues. Gardaí are in attendance. Can passengers outside of Bray please stay onboard the train. Staff and Gardaí are in attendance.
“Services will not be permitted to move until the line is clear. For health and safety reasons, passengers are not permitted on the line.
“Services have been suspended through Bray. Services will now be turned back to Dun Laoghaire. Dublin Bus will honour rail tickets in the affected area. Updates to follow.”
Many passengers took to social media to alert the railway company to issues with air conditioning on board.
One Twitter user said: “Passengers are boiling, no aircon, you should have planned better and people wouldn't have to resort to that. Absolutely ridiculous.”
Another wrote: “My mam is on that train and she’s called me panicking. Please open some doors or turn on the AC. She suffers with vertigo and will collapse.”
While another wrote: “Absolute disgrace on the DART today. No air con, people having panic attacks. No windows to open. Absolutely ridiculous.”