McGonagall's lips twitch when she's stifling a smile (this is stated several times in the text). Perhaps the same is true of Quirrell? In the second case, canon!Quirrell is responsible for both trolls that occur in the first book and at least according to Harry Potter Wikia is supposed to have a talent for using trolls. In the first case... I think what happened when Voldemort killed Harry's parents involved a sacrificial ritual (Lily sacrificing her life for Harry), so Quirrell might have found that particular idea ironic. Or maybe he's just amused by the idea of Harry using sacrificial rituals because of previous events.