Thieves have stolen seven gates from railway level crossings in the northwest.
The 4.3m by 1.5m steel barriers were taken from junctions at Clonloo, near Boyle, Co Roscommon and Culfadda, near Ballymote, Co Sligo.
They were noticed missing early on Sunday morning and Iarnród Éireann workers moved quickly to secure the unprotected openings.
Iarnród Éireann spokeswoman Jane Cregan said the removal posed a big threat to lives.
"Obviously it's a very serious matter to have a live railway with no barriers for pedestrians or livestock," she said. "This is something that we are taking very seriously, and indeed the gardai are investigating it.
"It could have caused a very serious incident and we are just happy that nothing happened."
Ms Cregan said the thieves would have needed a trailer because of the size of the gates, and the operation would have taken some planning. "It would have been quite a big job to remove these," she said.
While Iarnród Éireann has been targeted before for copper cabling, they have not had steel gates, which would be much less valuable, stolen.
The company said it was possible thieves wanted the gates for their own use or to sell on for use again or as scrap metal.
Gardaí in Boyle and Ballymote are investigating the thefts.