I figured the same about this morning. I guessed the train might as well stop at the Leixlips as there would have been another train ahead. Perhaps they made one train non-stop to Clonsilla and then expected us to pick up the Leixlip passengers. This would have worked a treat had the train been at normal strength. I would say that the ambulance service must have been very quick as we were stopped at Clonsilla for less than 10 minutes.
It wasn't really dangerously overcrowded in terms of it's not likely to kill anybody, but a 22K is not a nice place to be standing. I don't think the guards would get involved, but you've got to think that if you lost consciousness as a result of the overcrowding, it would be worth consulting a solicitor on whether or not you might have a civil case.