"The Pardoner's Tale" is filled with ironic statements. For instance, every time the three "rioters," as Chaucer calls them, use the term "brother" in any form to refer to one another the statements are ironic, since their relationship is anything but brotherly. In lines 122-124 Chaucer writes:
They made their bargain, swore with appetite,/These three, to live and die for one another/As brother-born might swear to his born brother.
The three greedy rioters later turn on each other and all three are killed. Of course, the greatest irony of "The Pardoner's Tale" is that the greedy pardoner uses a story about the evils of greed to separate naive listeners from their money and add to his own wealth.
I'm not sure which edition of the Tales you have, so I can't give you exact lines, but the Pardoner's prologue is actually an excellent place to look for situational irony.
Situational irony is "the difference between what is expected to happen and what actually does" (see link below).
The pardoner preaches, repeatedly, against greed (avarice); he says that greed is the root of all evil. YET, the purpose of his preaching is only to get the other pilgrims to buy pardons and relics from him so that he can have more money and fulfill his selfish desires to be rich. You would not expect a preacher, or a pardoner, to preach about the sin that they, themselves, commit.