D'Artagnan's Horse
At the start of the novel, when D'Artagnan leaves his family and his home, his father makes him promise to never sell his horse. Ironically, on his very first night in Paris, D'Artagnan sells the horse. This action subverts audience expectations about D'Artagnan keeping his promises, and implies that he is not invested in traditions or maintaining connections with his past. Now that he is in a new place and trying to reinvent himself, D'Artagnan is willing to discard any connections to his past and also ignore whatever advice he was given. Additional irony is created around the horse because D'Artagnan's father seemed certain the horse would help and support his son (he compares it to a faithful old servant) but it is actually the cause of the violent fight at Meung. In a sense, the horse ends up being a key way in which D'Artagnan finds himself initially entangled in the schemes of Milady and Rochefort.
D'Artagnan's First Duel
Early in the novel, D'Artagnan runs afoul of Porthos, Aramis, and Athos one by one, and ends up being scheduled to fight a duel with each of them. In a moment of dramatic irony, when D'Artagnan goes to keep his first appointment with Athos, Porthos and Aramis also arrive to act as Athos' seconds. The reader knows that D'Artagnan has planned duels with all three of the men, but the musketeers do not know this. The dramatic irony of the moment is comical and ends up lessening the tension. Shortly thereafter, when the Cardinal's guards arrive on the scene, D'Artagnan ends up fighting bravely alongside all three of the musketeers, marking the start of their friendship.
The Cardinal's Jealousy
Because of the Cardinal's status as a high-ranking member of the clergy and a man of God, it seems most plausible that he would be opposed to Queen Anne's affair because it is immoral and violates her marriage vows. Alternately, he could object on patriotic grounds, since she is effectively committing treason against France by pursuing an affair with a foreigner. Ironically, the reason the Cardinal is so angry about the Queen's affair is that he desired her for himself and was hurt when she rejected him. This rationale subverts audience expectations of why someone might object to an extramarital affair, and shows that the Cardinal is ultimately self-interested. He wants to shame her for doing something that he would have been happy to see her do if she had chosen him as her lover. His jealousy is also ironic because in theory he should be loyal to God first (and honor his vow of celibacy), and King Louis second. In fact, if he had the opportunity, the Cardinal would have been happy to betray both in pursuit of the beautiful Queen.
D'Artagnan's Bargain
When Monsieur Bonancieux first approaches D'Artagnan, he hopes for help because his wife has been abducted. Because a young woman is in danger, it seems like D'Artagnan would be compelled to help because of his honor and chivalry. Ironically, however, D'Artagnan's main initial motive is to avoid paying his rent! Monsieur Bonancieux is his landlord, and D'Artagnan is struggling with his finances, so the idea of free rent is very appealing. This irony subverts expectations of romance and chivalry. More typically, a young hero would be very wealthy (or totally disinterested in financial matters) and would act strictly from noble and pure-minded motives. Dumas uses this irony to subtly critique how idealism and romance narratives often obscure the practical details of everyday life.
