If you are asking for a person's opinion "What do you think?" you'd be more likely to use Cad a cheapann tú?. If you are asking "What are you thinking about?" (A penny for your thoughts), you could say céard air a bhfuil tú ag smaoineamh?, if you are asking if a person is thinking about something specific, you could say An bhfuil tú ag smaoineamh faoi dul go dtí an cruinniú? - "Are you thinking about going to the meeting?".