Great question! They're simiilar - both are often used to convey the opposite of what you really mean. An ironic comment is directed at a situation or event - it's not meant to insult people - for example, "nice day to go to the beach!" when it's pouring with rain. A sarcastic comment is directed at people and is meant to insult people - for example, telling someone "I love your painting" when in fact you don't like it at all.

Irony is more of a strange coincidence such as being thirsty in the middle of the ocean, a driving instructor causing a car crash, etc. Sarcasm is used to convey the opposite of what is said, often in a negative sense. For example if you had some bad luck and you said "oh, wonderful!" Or if you saw someone with bad clothes, you might say, "I see you dressed up"