Good point. It is possible that people may decide to stop in their homes and have a few beers rather than go somewhere that they are not allowed to smoke. So you are faced with more potentially drunk people smoking at home. Also if someone has been out drinking all night and unable to smoke, they are probably more likely to light up once they are at home and a bit drunk, whereas previously they might have had so many cigs in a night that they didn't have another one at home.
On the flip side, many people may stop smoking, which may counteract the other increase, and also people may actually drink less if they decide not to go to pubs to have a pint and a cig, so you may get less drunk people altogether.
Only statistics will show. Are there any statistics for the countries that have already implemented a smoking ban?