How is the ending of the poem "Casey at the Bat" ironic?

Irony is when there is a reasonable expectation that one thing will happen, and the complete opposite occurs. An example would be a firehouse burning down. One would assume a firehouse would not burn down because it is filled with firefighters whose job it is to stop fires.
The poem "Casey at the Bat" by Ernest Lawrence Thayer is a poem about a baseball game with an ironic twist at the end. The poem begins with little hope for the Mudville nine--unless Casey can get to the bat. The entire poem claims if Casey were to get up to bat, they would win the game. The idea repeats throughout the poem. In the second stanza the patrons claim: "'If only Casey could but get a/ whack at that—/ We'd put up even money now, with Casey at the bat.'"
Throughout the poem this irony is highlighted by excusing away Casey's first two strikes and by highlighting how assured both Casey and the audience are that he will clench their win. With the first strike, Casey claims it "isn't his style" and with the second strike, Casey didn't even swing the bat. "But Casey still ignored it and the umpire said/ 'Strike two!'". These instances build up the expectation that Casey is a confident baseball player who will win the game.
The first half of the poem builds up to the idea that if only Casey could get the bat, the team would win; however, it is Casey who ends up striking out and losing the game for the team. This creates irony in the final stanza. This fits the definition of irony because the poem leads the reader to believe that if Casey were to get to the bat, the team would win but, because of Casey, the team ends up losing the game--the complete opposite scenario of what was expected.

The poem “Casey at the Bat” by Earnest Lawrence Thayer (1863-1940) appeared in a Sunday edition of The San Francisco Examiner in 1888. Thayer was a well-known journalist who also wrote ballads for the newspaper. “Casey at the Bat” is an American classic about the beloved national sport of baseball. The poem describes a cheering and adoring crowd whom Casey, a much-admired baseball player, keeps in suspense when he refuses to swing at not only the first but the second ball that is pitched to him. He feels empowered by the ten-thousand eyes that are completely focused on his every move. Casey is very conceited and haughty—and overconfident. In a modern-day and almost comic way, he is guilty of the hubris, or pride, that brings down many a hero from Greek mythology. The irony at the end of the poem is that “Mighty Casey” misses the last ball and strikes out.