A couple of notable points of irony in the rhetoric of 1 John may be found in 1:7 and in 2:11. Irony entails a twist, in these cases an unexpected twist.
The irony in 1:7, of course, is in the statement, "and the blood of Jesus his Son cleanses us from every sin." Who among us would ever think to use blood as a cleansing agent? Instead, we think of blood as causing stains, nearly indelible stains, difficult to remove. One hardly thinks of blood as a cleansing agent. John, however, does. Of course, blood in 1 John 1:7 does not refer principally to the red substance that courses through our arteries and veins but rather blood is used by synecdoche for Messiah's bloody self-sacrifice, blood shed, the giving of his life on behalf of others..
The irony of 2:11 is in the statement "because the darkness has blinded his eyes." Ordinarily we think of bright light as blinding our eyes, at least momentarily (cf. Acts 9:1-9). John, however, expresses the unexpected, that "the darkness has blinded his eyes." Of course, John employs darkness, absence of light, metaphorically for being devoid of truth, of righteousness, of belief, of knowledge of God, of understanding the gospel. So accustomed is the person to absence of the light of truth and of righteousness that this person not only hates others but has no moral compass to discern proper direction but instead stumbles about, blinded by darkness.
Perhaps the irony of these two passages misses us because we have become so accustomed to reading the text without sufficient care and attentiveness or perhaps we have so frequently read the passage that it has little impact upon us because we assume that we know it. The ironies, however, are worth pondering, especially given themes in 1 John.