In Irish, the most common way to say "goodnight" is "Oíche mhaith" (pronounced "ee-hah wah"). This is the standard way of saying "goodnight" in Irish, and you will often hear it used in everyday conversation.
Another way of saying "goodnight" in Irish is "Dún do dhúnadh" (pronounced "doon duh doon-ah"). This is a more formal way of saying "goodnight" and is often used when addressing someone with whom you have a more formal relationship, such as an elder or someone you don't know very well.
A third way of saying "goodnight" in Irish is "Lámhacht leat" (pronounced "law-hukt lat"). This is a more colloquial way of saying "goodnight" and is often used between friends or family members. It is a more informal way of saying "goodnight" and is similar to saying "see you later" in English.
Overall, there are several different ways of saying "goodnight" in Irish, and which one you use will depend on the context and your relationship with the person you are speaking to.