Objection 1. It seems that irony is not a less grievous sin than boasting. For each of them is a sin through forsaking truth, which is a kind of equality. But one does not forsake truth by exceeding it any more than by diminishing it. Therefore irony is not a less grievous sin than boasting.
Objection 2. Further, according to the Philosopher (Ethic. iv, 7), irony sometimes is boasting. But boasting is not irony. Therefore irony is not a less grievous sin than boasting.
Objection 3. Further, it is written (Proverbs 26:25): "When he shall speak low, trust him not: because there are seven mischiefs in his heart." Now it belongs to irony to speak low. Therefore it contains a manifold wickedness.
On the contrary, The Philosopher says (Ethic. iv, 7): "Those who speak with irony and belittle themselves are more gracious, seemingly, in their manners."
I answer that, As stated above (110, 2,4), one lie is more grievous than another, sometimes on account of the matter which it is about--thus a lie about a matter of religious doctrine is most grievous--and sometimes on account of the motive for sinning; thus a mischievous lie is more grievous than an officious or jocose lie. Now irony and boasting lie about the same matter, either by words, or by any other outward signs, namely, about matters affecting the person: so that in this respect they are equal.
But for the most part boasting proceeds from a viler motive, namely, the desire of gain or honor: whereas irony arises from a man's averseness, albeit inordinate, to be disagreeable to others by uplifting himself: and in this respect the Philosopher says (Ethic. iv, 7) that "boasting is a more grievous sin than irony."
Sometimes, however, it happens that a man belittles himself for some other motive, for instance that he may deceive cunningly: and then irony is more grievous.
Reply to Objection 1. This argument applies to irony and boasting, according as a lie is considered to be grievous in itself or on account of its matter: for it has been said that in this way they are equal.
Reply to Objection 2. Excellence is twofold: one is in temporal, the other in spiritual things. Now it happens at times that a person, by outward words or signs, pretends to be lacking in external things, for instance by wearing shabby clothes, or by doing something of the kind, and that he intends by so doing to make a show of some spiritual excellence. Thus our Lord said of certain men (Matthew 6:16) that "they disfigure their faces that they may appear unto men to fast." Wherefore such persons are guilty of both vices, irony and boasting, although in different respects, and for this reason they sin more grievously. Hence the Philosopher says (Ethic. iv, 7) that it is "the practice of boasters both to make overmuch of themselves, and to make very little of themselves": and for the same reason it is related of Augustine that he was unwilling to possess clothes that were either too costly or too shabby, because by both do men seek glory.
Reply to Objection 3. According to the words of Sirach 19:23, "There is one that humbleth himself wickedly, and his interior is full of deceit," and it is in this sense that Solomon speaks of the man who, through deceitful humility, "speaks low" wickedly.
Source: Summa Theologica (New Advent)