I am around a lot alcohol with my work and generallythe courteous thing to do is offer me a drink as well, even though I am working, for me it is a easy throw of, just a smile and simply say no thanks..its easy.....and they get it, I am working....I didn't like to drink around clients anyway..
Not so close family and friends are different, I don't know why I feel I need to qualify my 'No thanks" but generally I do... I say things like "I have to get up early in the morning', 'I am driving', 'I am doing a detox', 'I have stuff I have to do later.' etc
There are only a few people that know I am an alcoholic.
I remember when I first quit smoking there were a few people that thought it was just hilarious to offer me a cigarette and blow smoke in my direction...I dont think that they are bad or stupid for doing that, just ill informed and not supportive.
I think a little bit of practise helps, prepare for the question....
I don't think he meant any real harm to you and probably thought he was somehow breaking the ice with a dig at the irish..but its so easy to get caught of guard with these statements because we can feel so fragile and sensitive around the topic..
It gets easier.