Authorities say a masked gang has stolen four stuffed rhinoceros heads containing eight valuable horns from the warehouse of Ireland's National Museum.

Police say three men raided the storeroom in Swords, north of Dublin, on Wednesday night and tied up the lone security guard. He later freed himself and raised the alarm.

Nigel Monaghan, keeper at the National Museum's natural history section, says the four heads were removed from display last year and put into storage specifically to safeguard them from thieves.

He says the eight horns could be worth around €500,000 ($650,000) on the black market based on their weight.

Some species of rhino are being hunted to the verge of extinction because their horns, in powdered form, command high prices for use in traditional Asian medicine.