Thieves stole antiques worth €100,000 from the home of Lord Henry Mountcharles while he was in the house, and escaped without being heard.
Lord Mountcharles's home is Beauparc House on his estate in Slane, Co Meath. It is close to Slane Castle, which he also owns.
The culprits had earlier broken into a gate lodge on the estate and stolen firearms, mainly rifles, before going to the main house.
The burglaries happened on Friday, between 8 p.m. and 9 p.m.
A Garda spokesman confirmed that Lord Mountcharles was at home at the time.
"Lord Henry was there but he was unaware that the burglary was taking place and the thieves got away," he said.
The property stolen includes a double-hinged 18-carat gold rectangular vesta holder, marked 1855; a rectangular gold snuff box dated 24/12/1762; an 18-carat two-coloured gold snuff box, dated 13/5/1831; a George 1V 18-carat unmarked gold ring; a rectangular engraved tin box, dated 1850; a silver and enamel Garter badge; and an 18-carat ladies' gold keywound pocket-watch, dated 1850.
Gardaí have appealed for anyone with information to contact them at Slane, on 041-9824202, at the Navan Garda station, on 046-9079930, or on the Garda confidential line on 1-800-666-111.