The derivation of "bull" in this sense is unclear. It may be related to
Old French boul "fraud, deceit, trickery",
Icelandic bull "nonsense",
Middle English bull "falsehood", or the verb
bull "befool, mock, cheat". As the Oxford English Dictionary points out, the epithet "Irish" is a more recent addition, the original word
bull for such nonsense having been traced back at least to the early 17th century. ==Examples==