Irony is a form of humour that has been around for a long time. The dictionary defines "irony" as a "rhetorical device, literary technique or situation in which there is an incongruity or discordance that goes beyond the simple and evident meaning of words or actions."
Maybe looking at a few signs will easier to explain. See below …
P.S. For those who are keen for more, check out the Friends of Irony web site.