Farrah Abraham just pled GUILTY in her Nebraska DUI case -- and as a result, she's been ordered to stay ALCOHOL-FREE for six whole months, TMZ has learned.

We broke the story ... the former MTV star turned porn queen had struck a plea deal with prosecutors -- and got probation instead of jail stemming from her March arrest.

As part of the deal, Farrah was sentenced to 6 months probation, a $500 fine, and was ordered to install an ignition interlock device in her car -- one of the machines that requires the driver to submit to a breath test before the engine will start.

The judge in the case also required 22-year-old Farrah to not consume one single drop of alcohol during her probation.

Farrah was initially charged with disorderly conduct and refusing to take a field sobriety test -- but those charges were dropped as part of the plea deal.

Congrats?