Procedure is normally to run the points back and forward a few times to clear the issue, then get a member of station staff to go trackside and ensure there isn't anything obstructing the points and follow the proceedure
The exact procedure for the exact points in Dun Laoghaire
https://youtu.be/cc3r_q5Vgrs?t=274
If I was a betting man, one of two things happened next
1. A stretcher bar fails and the gauge wideness just enough for the train derail, there was a new stretcher bar installed post incident
2. The lead axle wheel strikes the wooden scotch and lifts it such that the second axle climbs over it