While there may be various types of irony in Elliot's Silas Marner, Elliot emphasizes situational irony. Situational irony is defined as situations in which events occur that seem oddly appropriate or painfully inappropriate. An example of the first is the attitude of the towns marriageable young women noted in Part I, Chapter 1. These young ladies are first piqued that Silas (an eligible, though peculiar bachelor) doesn't socialize in the town and beg one of them to accept him, but they soon say they wouldn't "marry a dead man come to life again." This is ironically oddly appropriate because Silas has no intention of socializing or courting. An example of an ironic situation that is painfully inappropriate is Silas's exile from town following Silas's curing of Sally Oates by use of herbal remedies. The other two kinds of irony are verbal irony (spoken or written) in which someone says one thing that clearly has an undertone of meaning another thing. An example that comes readily to mind is saying "Thanks for helping me find my glasses" to the the person who is sitting on them and not helping you find your glasses. The other kind of irony is dramatic irony in which the reader (or audience) knows an important fact that the character does not know. The reader/audience sits by, sometimes aghast, sometimes amused, sometimes in horror, while the character behaves wrongly based on wrong or incomplete information.
Elliot, like all writers, uses irony to clarify her characters qualities (e.g., someone heading bravely in to a situation versus kicking his heels and loudly resisting action); to clarify the connection of events to themes (e.g., being exiled for doing good); to dramatically call attention to a character or a situation related to thematic concerns (e.g., Silas not caring about courting and how that indicates something about him psychologically); to clarify the themes of the novel (e.g., Nancy Lammeter's involvement with a married man illustrating Elliot's theme of moral order.) So in summary, irony, in its three forms, causes the reader to do a sort of double-take and attend more strictly to the point the author wishes to make about a character's traits, a situation or the themes of the story. In addition, many messages can be effectively conveyed through irony, which invokes emotions ranging from humor to fear to sympathy to horror in the reader, that given a didactic, serious presentation would read like a diatribe or a sermon and therefore not effectively make the author's point(s).