Gardaí are investigating the theft of Rhino heads and horns from the National Museum Archives in Swords, Co Dublin.
Three masked men entered the building and tied up a security guard on duty at around 10.40pm.
The three loaded Rhino heads and horns from the building into a large white van.
The raiders were in the building for approximately one hour.
The security guard, who was uninjured, later freed himself and raised the alarm.
The value of the property stolen is estimated to be around €500,000.
The crime scene has been sealed off for forensic technical examination.
Gardaí said the gang appeared to know where to find the heads and horns in a building that is the size of two football pitches.
Anyone with information is asked to contact Swords Garda Station on 01-6664700.