An Irish bull is a ludicrous, incongruent or logically absurd statement, generally unrecognized as such by its author.
[1] John Pentland Mahaffy, Provost of Trinity College, Dublin, observed, "an Irish bull is always pregnant", i.e. with truthful meaning.
[6] 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.
[1] By the late 19th century the expression Irish bull was well known, but writers were expressing reservations such as: "But it is a cruel injustice to poor Paddy to speak of the genuine 'bull' as something distinctly Irish, when countless examples of the same kind of blunder, not a whit less startling, are to be found elsewhere."
The passage continues, presenting Scottish, English and French specimens in support.