“illegal criminal conspiracy” to do what?
There are numerous links to the case, one of which is just a few paragraphs succinctly outlining the case, Federal Society of Journeymen Cordwainers. Subsequent rulings of that era found labor unions to be engaged in illegal restraint of trade in what the courts characterizes as "combinations". The Danbury Hatters case is another interesting one which found that secondary boycotts (a favorite tactic of union goons nowadays) violated the Sherman Antitrust Act.
As it was then, at the time of the Founders, it is so now, unions are anti-liberty and opposed to the free market. Sadly, leftist courts have undone the constraints once placed on union thugs during the time of the Founders.