Apparently it comes with a preset factory default detonation timer of 10 minutes. The timer can be remotely set to something else.
The Irradiator 3000 is similar to, and looks like, the radiation bomb initially set off to kill the troggs invading Gnomeregan during the troggs' initial invasion prior to Thermaplugg's takeover.
The World of Warcraft manual, page 174, says "At the command of the High Tinker Mekkatorque, the gnomes opened the pressure valves of their giant, grinding machines and released toxic radiation throughout the city." Also, the official site here says "In a desperate attempt to destroy the invading troggs, High Tinker Mekkatorque ordered the emergency venting of the city's radioactive waste tanks."
Neither version was mentioned in World of Warcraft prior to Operation: Gnomeregan. The Gnome introduction movie rewords the passage mentioned in the manual to avoid any specific reference to the radiation.
While the Irradiator 3000 does have tanks of some presumably radioactive liquid attached to it, it is difficult to reconcile "venting tanks" with "detonating a bomb", even in a gnomish mindset.