Irony

When something is ironic, it has a grimly humorous or paradoxical twist, as if the universe were playing a wicked practical joke. Thus, it is ironic if a speeding car crashes into a "drive carefully" sign, or if someone named Joe Friendly turns out to be a serial killer.

Do not use irony or ironic to describe a simple coincidence: It's so ironic that our birthdays fall on the same day. No irony there; it's merely coincidental.