How would you say what was the temperature last night?
cad a bhí an teocht aréir
Another possibility would be Cár an teocht aréir? (cár is the past tense form of cá ; they can mean “what” as well as “where”), though that question might be closer to “which” than “what”.
EDIT: Expanded upon caveat.
Síolta, tá sé i gcónaí deas rudaí nua a fhoghlaim :)