Mountpanther, in Newcastle, boasts scenic views across the coastline and Mourne Mountains, however, no buyer has been found since it went on the market for £8m in 2007.
Ulster Property Sales said it now thinks the £5m price tag for country retreat and and 170 acres of farm land is more realistic.
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"Prices have dropped from the bottom of the market to the top and just because Mountpanther's such a special place doesn't make it any different," UPS's Joanne McKee said.
"We feel we have put a realistic price on it."
Famous guests to the once opulent 18th Century mansion included the late Princess Margaret and her former husband Lord Snowdon, as well as Dynasty star Joan Collins.
However, the grand home now lies derelict. But UPS has been keen to stress it offers "huge potential" to developers.
"We've had no offers yet, but we're working very closely with a couple of interested parties and we feel we're getting to the stage where a sale could be made soon," said Ms McKee.
At one time a safari park was housed on the site, but it now looks more likely a golf course or leisure resort could take its place.
(PR/BMcC)
