Symbolic motifs, tragic, comic and ironic elements in the novel.
The Marabar Caves cover that question. At first, I thought the question was going to involve finding a different, specific thing/event for each element. But the caves and the echoes that come from them provide plenty of symbolism and lead toward the other elements.
The caves themselves represent an alien side of nature. The caves are very old and have a mixture of beautiful and menacing features. There's an overall sense of mystery and mysticism around those caves. But it is the cave's echo that unsettles Mrs. Moore and Adela the most. No matter what sound is produced, the caves seem to echo back the same sound. A "boum" is the only sound that the caves ever return.
The echo is symbolic of the Hindu "oneness" of everything. One spirit, one world, all nature and people are connected, etc. It's a nice way of thinking about things. Very transcendental. It's also incredibly tragic. If all things are the same thing, then there are no differences. No separation of good from evil. No differences in values systems. When Mrs. Moore eventually realizes this logical conclusion, it causes her to have a falling out from her belief system. Additionally, she ceases to care about human relationships. That's tragic.
The irony in the story, is a large overarching plot irony. It's not a single event. Mrs. Moore and Adela both came to India and wanted to see "the real India" as a way to enhance their learning, beliefs, values, knowledge, relationships with people, etc. The irony is that the real India, and its Hindu influences, are what cause Mrs. Moore to no longer care about much of anything, especially people.