De Bruin had appealed to the Court for Arbitration in Sport after she was suspended last August by Fina, the international swimming federation, for tampering with a urine sample from an out-of-competition test conducted at her Kilkenny home in January last year. According to the testing officers, De Bruin's sample smelled strongly of whiskey, and subsequent laboratory analysis discovered a high level of alcohol in the sample.
Under international swimming rules, manipulating a urine sample carries the same four-year penalty as a positive drug test for anabolic steroids.
De Bruin, who has always denied the charges against her, is married to the former discus thrower, Erik de Bruin, a former Dutch champion who was banned from athletics for four years after testing positive for drugs. Her case was heard at the beginning of last month by a panel of three experienced sports lawyers, including the Briton, Michael Beloff QC.
Uniquely for a CAS hearing, De Bruin's case was heard in public, at her own lawyer's request. Thus, it became known that three samples, taken between November 1997 and March 1998, had shown signs of the body-building drug androstenedione.
De Bruin's ban, however, is for tampering, rather than for drug use. A statement from CAS's Lausanne offices last night read: "Based on the facts of the case, and the evidence before them, the arbitrators were of the opinion that Fina had convinced them that Smith-de Bruin was the only person who had the motive and opportunity to manipulate the samples."
Gunnar Werner, the secretary of Fina, said: "Manipulation is a bad crime. It is more or less two abuses in one - manipulating samples to cover up something else."
The CAS decision should be binding on both De Bruin and Fina. De Bruin's only possible appeal might be to a Swiss federal court or the European Court of Justice. De Bruin's Dublin lawyer, Peter Lennon, was unavailable for comment.
The decision was taken as vindication for the husband and wife team of doping control officers, Al and Kay Guy, whose role in the affair had been placed under close scrutiny by De Bruin's defence. "It's a great relief because we felt we were on trial," Kay Guy said. "It marks the end of what has been a very difficult year."
Although De Bruin had always been an outstanding swimmer by Irish standards, she did not make an impact internationally until four years ago. Her best result in a career stretching back to the 1988 Seoul Olympics was to make a world championship consolation final.
Then, in 1995, two years after De Bruin, her then boyfriend, had taken over her coaching, Smith became the first Irish swimmer to win a European title. The following year, in Atlanta, she astonished the swimming world when - at 24, a veteran by swimming standards - she won gold medals in the 400 metres freestyle and 200m and 400m individual medley events, plus a bronze in the 200m butterfly.
De Bruin maintained her innocence, claiming she was tested regularly and often. When the Fina testers arrived at her home last January, it was their third attempt to take a sample from her after two failures.