A collection of State-owned art and antiques has been moved from a private storage depot in Co Kildare to the National Museum of Ireland’s storage facilities for safe-keeping following the revelation that some items had been stolen.

A spokesman for the Garda said the force was “investigating the theft of a number of items taken from a storage unit in the Naas area on behalf of the Department of Arts, Heritage and Gaeltacht” and that enquiries were “ongoing”.

The Department only became aware of the theft when paintings from the collection were spotted for sale in an auction last October. It is not known how or when they were stolen.

The department would not reveal how many items are in the collection nor how many are missing but the Minister, Jimmy Deenihan said “this is an extremely serious matter” and that he had “instructed” his officials “to give every possible assistance to An Garda Siochána in order to help return these items to the State”.

When he was informed of the theft, the Minister requested that officials carry out an inventory of “all artwork, furniture and other items in storage” and “a physical check on all of these items”.

That work has been completed and, in a statement, the department has confirmed the “loss of a number of items” but declined to provide any details and said “inventory details are not being released while an active Garda investigation is underway”.

The collection consists of art and antiques from Killarney House, in the grounds of the National Park in Co Kerry, the ancestral home of the earls of Kenmare which was acquired by the State over a decade ago. Because the house required conservation work, the contents were put into storage. It was intended that the art and antiques would eventually be returned to the house and go on public display. However the planned restoration of the house has been repeatedly delayed and no date has yet been announced for its opening.

Killarney House is known to have been furnished with a significant collection of paintings, antique furniture, china, silver and other fine art collectibles numbering hundreds of items.

The theft of items from storage came to light last year when two paintings (portraits of the Earl and Countess of Kenmare) in an Adam’s auction at Slane Castle in Co Meath were spotted by a public servant who realised that they were part of the Killarney House collection.

The portraits had been consigned to Adam’s by a Dublin art dealer who had bought them at a Bonhams auction in London, where they had been catalogued as “English School” but which he recognised as the work of Irish artist Hugh Douglas Hamilton. The paintings were withdrawn from the Adam’s auction and are now in The National Gallery of Ireland for temporary safe-keeping.

Speaking to The Irish Times on condition of anonymity, the dealer said he believed that the thieves who stole the paintings from the storage depot probably didn’t know their importance or value, and he didn’t blame Bonhams, who were “very honourable people”, for not recognising Hamilton’s work. He said Irish portraits could “slip through” in London valuations. The dealer said “it was sheer luck that an Irish buyer bought them and brought them back to Ireland”.

James O’Halloran, managing director of Adam’s auctioneers in Dublin said the firm had given “a detailed statement to the gardaí” and that when Adam’s had been preparing the catalogue for the auction they had checked with the Art Loss Register (an international database of stolen and missing art) and “they were not listed”.

Separately, a painting sold at Sotheby’s in London in 2010 is also believed to have come from the Killarney House collection.

Both Bonhams and Sotheby’s, two of the world’s most prestigious fine art auctioneers, have confirmed that they are helping with the garda investigation.

Rumours in the art market about other items from the Killarney House collection being offered for sale cannot be substantiated.