Even private-equity guys no longer have an appetite for trophy real estate. Quinlan Private, the $15 billion fund run by Irish hotel mogul Derek Quinlan, put its East 64th Street townhouse office on the market for $36 million. The 25-foot-wide mansion was home to the New York Observer until 2004, when owner Arthur Carter sold it to a Russian developer for $9.5 million. After a renovation, Quinlan bought it for $18.7 million the next year. “It’s just too big for us,” says Quinlan director Bill Daly. “We have 10,000 square feet for five people.” But doubling the asking price in two years? “It’s ambitious,” allows Stribling’s Shel Joblin, who sold the building to Quinlan. Quinlan’s broker at Sotheby’s had no comment.