Do they accept Mir cards on Samui and can you withdraw money?

No, of course they don’t accept them, and you also can’t withdraw money. Mir cards, due to sanctions, do not work almost anywhere in the world except for a few countries, but Thailand is not among them. Therefore, on Samui and nowhere in Thailand at all can you pay with a Mir card or withdraw money from it. For more details on everything you need to know about currencies, money, and plastic cards in Thailand, read here - https://nashaplaneta.net/asia/thai/valuta-dengi-v-tailande

Where to go in Thailand for the first time?
Do I need a category "A" license to rent a motorbike?
Can I exchange rubles in Phuket?
Follow the link, all these questions are discussed in the chat - https://t.me/thailand_travel_chat