Is it possible to drink alcohol while on xenical tablets?

Yes. There is no specific contraindication of alcohol use together with xenical. However, since the purpose of xenical is weight loss, you need to know that alcohol has almost as many calories per gram as fat, so a standard drink has over 100 calories just form the alcohol; beer and wine have additional calories from other carbs in them. The main thing is to cut back on fat, which will cause diarrhea.
Yes. There is no specific interaction with alcohol, however, xenical and alcohol are absorbed and metabolized in the liver and could cause worsening of liver disease if used together.