I have a "inguinal hernia" and I have to take a DOT physical. No pain, discomfort and no medication. Will this fail me on my DOT exam?

Inguinal hernias are extremely common in men. Unless you are in a job in which exertion might worsen the hernia, it should not be an issue.

However, be aware that these are quite easy to repair. My husband had it done last year, even though he wasn't experiencing any problems, just because these hernias can worsen over time. My brother's had it done, too.

Yes. Yes it will.

A hernia should not be relevant to a medical in respect of driving (except for some lorry/truck drivers who have to lift heavy items in loading/unloading the vehicle.

A hernia is only relevant in jobs where a person is required to perform a lot of lifting or carrying heavy objects.    Failure of DOT exams are generally for conditions that might make a person dangerous when driving a vehicle - such as seizures or blackouts. 

You're lucky. The remedy for this 150 years ago was castration.