Sources

Foley 1820 Thomson

“He built an elegant house on some high grounds not far from the parish church, and gave it a fine high-sounding name, which I forget, but the Rothesay folks were not satisfied with the name, and in spite of him always continued to call it “The Folly” It was afterwards purchased by Lord Bute, and occupied by Colonel Moore, who, wisely humouring the whim of the Rothesayanians, called it “Foley House.”” Glasgow past and present 1851, 134

Whether this comment is true or not, the derivation could be considered obscure.