When is drinking alcohol a mortal sin?
Over drinking to the point that the drinker loses the use of reason, i.e. not having the freedom to think responsibly and follow through on reasonable conclusions. But in order for this to be gravely sinful, the act has to be deliberate, not something that happened by accident or something. If this freedom is lacking, then the act of intoxication would be a venial sin.
But another thing to remember is that if any mortal sins are committed while drunk, even if they were not freely willed in themselves, they were willed in their cause. This means the drinker chooses to be drunk, and this drunkenness is the primary cause of said sinful act. So the drinker is still responsible for those acts as though they were chosen deliberately.
Then another point to remember is that damaging your health may also be sinful. Since you are charged to take care of the body you have received. Exposing yourself to an unnecessary danger that might cause you to lose your life is sinful. The gravity will depend on the circumstances. Drinking is good in moderation but can cause grave harm when abused.
Drinking and driving is another potentially grave evil that can cause the loss of lives. As we said above, exposing yourself (and in this case others), to unnecessary grave danger is also sinful. The blood alcohol limit established by law in most countries is usually low, but they are set because it’s at that point that some reflexes may be impaired which will make driving unsafe. So, ignoring these can be a grave matter even if the driver does not fit what you’d classically call “drunk”.