How is irony expressed in the story “One Thousand Dollars”?

There are three instances of irony in O. Henry's story "One Thousand Dollars":
It is ironic that Young Gillian does not grumble about inheriting only one thousand dollars when he has been entirely dependent upon his uncle's wealth.
It is ironic that Young Gillian is unselfish in his efforts to spend the money, rather than spending it on himself as has been his habit.
It is ironic that when seemingly self-gratifying Gillian learns that his act of being unselfish will earn him a larger inheritance, he falsifies his report so that the young woman he loves will be given the money instead.
1. After Gillian is told by the lawyers that he must "render...an account of the manner of expenditure of this $1000 as soon as you have disposed of it," he goes to the men's club and asks Old Bryson what he should do with the money, and Bryson suggests that he buy Miss Lotta Lauriere, an actress, a diamond necklace.
2. When Gillian asks her if she would like a pendant for a thousand dollars, Miss Lauriere alludes to another actress she knows who has received a necklace that is worth much more. Defeated, Gillian leaves, but he asks the cab driver what he would do with $1000. When he does not like the man's answer, Gillian then asks a blind man how he would spend the money. The man shows Gillian a bank deposit book with more than a thousand dollars in entries; Gillian returns the book and re-enters the cab.He then goes back to the law offices and inquires if Miss Hayden has been left anything besides a ring and $10. Lawyer Tolman replies in the negative. So, Gillian has the cab take him to the house of his dead uncle where Miss Hayden is seated, writing letters in the library. Gillian tellsher that the old lawyers have found a codicil to the will and she is to receive one thousand dollars. Further, he tells her that he was driving this way and lawyer Tolman asked him to bring it to her. Blanching, Miss Hayden can only utter, "Oh!" and repeat "Oh!" Gillian declares his love for Miss Hayden, but she responds, "I am sorry," and takes the money.
Going into the next room, Gillian writes out the account of his expenditure:

Paid by the black sheep, Robert Gillian, $1000 on account of the eternal happiness, owed by Heaven to the best and dearest woman on earth.

Slipping this into an envelope, Gillian departs.
3. Gillian returns to the offices of Tolman & Sharp with his written accounting of how he spent the $1000. He tosses the white envelope upon the table before Mr. Tolman, saying, "You will find there a memorandum, sir, of the modus operandi [method] of the vanishing of the dollars."
Without looking in the envelope, Mr. Tolman calls his partner and together they explore the inside of a huge safe. Finally, they bring out a very large envelope sealed with wax; then they explain that there is a codicil to his uncle's will. When Bobby Gillian has spent his $1000, then this document is to be read. The lawyer reads,

"If your disposal of the money in question has been prudent, wise, or unselfish, it is in our power to hand you over bonds to the value of $50,000, which have been placed in our hands for this purpose....But, if....you have used this money as you have used money in the past—I quote Mr. Gillian—in reprehensible dissipation among disreputable associates,—the $50,000 is to be paid to Miriam Hayden....I will examine your account in regard to the $1000....I hope you will repose confidence in our decision."

As Mr. Tolman reaches for his envelope, Bobby Gillian grabs it first; he tears it into strips and places it into his pocket. He tells the old gentlemen they need not bother to read his account of his itemized bets. "I bet the thousand dollars at the races. Good-day to you, gentlemen." Mr. Tolman and Mr. Sharp shake their heads in disapproval as they watch Gillian depart, whistling happily in the hallway as he goes toward the elevator.