There are all different types of irony, "situational irony" being the most commonly referred to, and "cosmic irony" being the second, in my experience.
Situational irony is "a discrepancy between the expected result and actual results when enlivened by 'perverse appropriateness'."
An example of this is when John Hinckley attempted to assassinate President Ronald Reagan, all of his shots initially missed the President; however a bullet ricocheted off the bullet-proof windows of the Presidential limousine and struck Reagan in the chest. Thus, the windows made to protect the President from gunfire were partially responsible for his being shot
Cosmic irony stems from the notion that the gods (or the Fates) are amusing themselves by toying with the minds of mortals, with deliberate ironic intent. Closely connected with situational irony, it arises from sharp contrasts between reality and human ideals, or between human intentions and actual results.
An example of this is in 1974, the Consumer Product Safety Commission had to recall 80,000 of its own lapel buttons promoting "toy safety", because the buttons had sharp edges, used lead paint, and had small clips that could be broken off and subsequently swallowed.
All of the above knowledge (and more!) can be yours, a bargain at $0, at
http://en.wikipedia.org/wiki/Irony
Suffering fools begrudgingly,
-Hunter