Aside from some interesting Chinese historical use of explosives, until now the earliest use of an IED that I could find in records was at the siege of Pskov in 1581.  The city of Pskov was being besieged by Stephan Bathory, who had been elected King of Poland.  Bathory’s troops were Polish, German, Hungarian and Scottish.  Bathory had an IED made in the form of a jeweled casket, by an IED maker called Johann Ostromecki that was sent to the Russian defender Ivan Petrovich Shujski.  The casket was sent to Shujksi ostensibly by a freed Russian prisoner. The casket, “booby-trapped”, exploded when opened by some of Shujski's companions ,killing them but not its intended target.

Other historical use of IEDs from around the same time include roadside IEDs being used to ambush invading Spanish troops in Holland (I have a copy of great engraving showing a multiple IED attack from around the 1580s) and English use of “exploding” fire ships also against the Spanish%u2026 and evidence of an Italian engineer who seemed to be designing a range of innovative explosive devices for the English around this time.

However, my research over the past few days has uncovered perhaps earlier use of IEDs.  The key technological development within the confines of my definition, is the invention, around 1500, perhaps by Leonardo da Vinci, of the flintlock/wheelock mechanism. Such a mechanism was actually first made somewhere around 1510-1520. This invention provides the opportunity to initiate gunpowder charges at a distance by means of a spring to release the mechanism, by pulling a string.  Using a clock to initiate the flintlock also developed about this time and again I have found an interesting diagram from some time in the 1500s allegedly showing a clock initiated IED.

from http://www.standingwellback.com/home/2011/9/5/historical-use-of-ieds.html