I've been told that many Folans changed the name to Foley after coming to the US. Maybe it sounded more Irish? My wife's a Folan, her Dad tested his paternal DNA and matches several Foleys at varying degrees of relatedness at Family Tree DNA.
An Irishman told me that Folan and Foley are separate surnames in Ireland. That Foley translated as a type of pirate or robber in ancient days, so most Foley aren't related from a single clan founder like many other Irishmen.
Maybe that source saying Folan and Foley both are O Cualain is simply alluding to the fact that many Foley were once Folan (though obviously many are not).
Tim Fichtner for Marty Folan