Does a year of entrepreneurship in an unrelated field affect an MS application? I did my Bachelors in computer science. I have been working for 1.5 years in a reputed software firm working on distributed systems, information retrieval and such fields.

I want to apply for MS in CSE (distributed systems or ML) next year but before that I want to start my game development company which has been a dream. It might also help me earn a lot of money which I will need to study further. Game development is largely software engineering, but has nothing to do with distributed systems or ML. If my game development start-up fails I will happily do the MS right away.

Will switching from my current "related" job to a somewhat unrelated field trigger an alarm? Am I at a risk of getting rejected?

How can I patch things up if I have to after my game development gig?
I think you are forgetting that you do not necessarily need to self-finance your graduate study.  A teaching assistantship would take care of your tuition and would give you a stipend to live on (frugally).  Teaching assistantships are not hard to come by in the field of CS -- as compared to some other fields, because universities generally need a fleet of TAs to grade and/or teach the undergraduate classes.

You may want to consider putting in some applications for grad school this coming December - January.  You can always quit your current job after collecting a strong recommendation from your current supervisor.  Then you would have February - August to try out your entrepreneurial idea and see how it goes.  If it's taking off and you're really enjoying it, you can always cancel (or possibly postpone) the grad school plans.