Masked men have stolen four stuffed rhino heads from Ireland's national museum.

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

Nigel Monaghan, keeper at the museum's natural history section, said it had never before experienced such a theft but had worried that the rhinos would be targeted.

He said the four heads – three of black rhinos from Kenya, one of the virtually extinct white rhino from Sudan, all killed more than a century ago – were removed from display last year and put into storage to safeguard them from thieves.

He said the eight horns could be worth about €500,000 (£430,000) on the black market based on their weight.

Three of the five species of rhinoceros in Africa and South Asia have been hunted to near extinction because their horns command exceptionally high prices for use in traditional Asian medicine, chiefly in China and Vietnam, where the powdered horn is marketed as an aphrodisiac and even a cure for cancer.

The horns are made of keratin, a fibrous protein that is the building block for skin and hair. It has no documented medicinal value.