What is the mechanism of action for ethanol to act as central nervous system depressant?

CNS depressant. I don't know that anyone has an absolute lock-down on this mechanism, but you can look up what's known in goodman and gilman's pharmacologic basis of therapeutics. Gaba-a receptors are inhibitory in the brain, and their function can be enhanced by alcohol as well as sedative-hypnotic agents including benzodiazepines. Several types of ion channels in the brain are sensitive to ethanol also.
Very complicated. If you drink more than your liver can detoxify, the alcohol will "numb" your cortex causing disinhibition and sometimes euphoria. If it reaches your brainstem, the alcohol depresses the nerves that control involuntary actions such as breathing, heartbeat, motor regulation, temperature control, and the gag reflex. Severe alcohol poisoning can be fatal.