Irony refers to a figure of speech where someone or something says/ does something that contrasts sharply or is the opposite of the message they want to convey.
In this instance by imitating the way a sloppy person eats spaghetti, Sally is trying to demonstrate the opposite- showing that good table manners are desirable.
Another example of an ironical statement would be, "Brilliant, I'm really looking forward to hours of study this weekend while my friends are going to the beach without me."
