For me, the irony is not that the old man that shows up on the beach one day in "A Very Old Man with Enormous Wings" (by Gabriel García Márquez)—with the potential to be an angel—and is treated shabbily because of his appearance. That is not a surprise. The irony is that even as they see themselves as people of faith—as does the town priest—they are all blind to the fact that angel or not, the old man is God's creature.
There is a strong parallel here with the story of Christ—who was rejected by his own people because they had anticipated a Messiah of great power who would destroy the Romans, their oppressors. Jesus comes, rather, as a peacemaker; because he is not who they expect him to be, the people turn on him and kill him.
While the people in this story stop short of killing the old man (though it does enter into conversation), they fail to treat him like a heavenly creature because he defies their expectations.
Alien to the impertinences of the world, he only lifted his antiquarian eyes and murmured something in his dialect when Father Gonzaga went into the chicken coop and said good morning to him in Latin. The parish priest had his first suspicion of an impostor when he saw that he did not understand the language of God or know how to greet His ministers. Then he noticed that seen up close he was much too human...
Ironically, this is very much how the Pharisees and high priests saw Christ—he was too human, not the supernatural figure they hoped for.
Biblical examples that appearances are often deceiving abound. Judas, one of the Apostles, betrayed Christ. God appeared to several notable Old Testament figures as a common man (e.g., Abraham).
A verse from the Bible's New Testament cautions...
Do not forget to entertain strangers, for by so doing some people have entertained angels without knowing it. (Hebrews 13:2, NIV)
This verse is lost not only on Father Gonzaga (who really should know better if anyone should), but also on the townspeople:
...when they went out into the courtyard with the first light of dawn, they found the whole neighborhood in front of the chicken coop having fun with the angel, without the slightest reverence, tossing him things to eat through the openings in the wire as if he weren't a supernatural creature but a circus animal.
Our "angel" is not without angelic qualities. It is noted...
His only supernatural virtue seemed to be patience.
However, Pelayo and Elisenda become quite wealthy because of the presence of the "old man," a miracle at most; a blessing, at the least.
There is also a certain irony in the greater loss the community suffers: in its inability to see the old man as an angel, they are unable to see that which is divine in themselves and others simply because their expectations are very different than what they see in the old man. Ironically, dismissing the seemingly worthless old man they harm themselves.
While these people act like they are guided by God and Biblical teachings, it is ironic that they allow themselves to be "blinded" by what they see. The old man seems unimportant, but the people's rejection of him is a rejection of God—regardless of whether this is an angel or not:
I tell you the truth, whatever you did for one of the least of these brothers of mine, you did for me. (Matt. 25:40 NIV)
The life of Christ is lost on these people. Irony serves to demonstrate that while people search for the divine, they miss it when it stands right before them.