The 55-year-old, his wife and son were at home when thieves broke into Castle Hyde, on the banks of the Blackwater river, early on Monday evening.It is understood the horn was cut off a stuffed rhino head on display in a safari room within the 18th century castle, which was the ancestral home of Ireland's first president Douglas Hyde.The burglary was reported to gardaí at about 6.20pm.Rhino horn can be sold for tens of thousands on the black market and is particularly prized by those involved in traditional Asian medicine, despite being proved ineffective.While no arrests have yet been made regarding the latest theft, it is believed it could be linked to an Irish traveller gang suspected of carrying a number of similar robberies across Europe.International policing agency Europol has previously issued a threat notice about the gang.To source and acquire rhino horns, the group has targeted antique dealers, auction houses, art galleries, museums, private collections and zoos, resorting to theft and aggravated burglary where necessary.EuropolLast year, four rhino horns were stolen from a storage unit used by the National Museum of Ireland in Dublin - despite having been taken off display due to concerns that they might be taken.Flatley has, however, come under fire over the break-in at his home, as animal rights activists have been angered that he would own such an item in the first place.In an open letter to the star, one group called ARAN said: "Has it set in or even occurred to you that neither you nor those who stole the horn are the rightful owners?"That horn once belonged to a sentient living animal whose very species is in serious trouble right now due to poaching and hunting."The animals are literally on the brink of being wiped off the face of the planet."The letter added: "If the rhino horn is returned to you, may we suggest that you donate it to ARAN so that we can publicly burn this item of immense cruelty."CCTV footage from Flatley's home will be examined as part of the investigation into the theft of the rhino horn.