Two retired LIRR employees admitted Monday they were part of what prosecutors contend was an extensive scheme to falsify disability claims, which officials estimate could have cost taxpayers $1 billion. Testifying as witnesses for the prosecution in federal court in Manhattan, former LIRR benefits manager Regina Walsh, 65, of New Hyde Park, and retired conductor James Maher, 60, of Florida, said they exaggerated their disabilities and implicated two of the three defendants on trial facing charges they were key facilitators of the fraud. Both Walsh and Maher said they pleaded guilty to charges stemming from their involvement in the case. Walsh, a 30-year LIRR veteran who retired in 2006, said she faces 6 to 12 months in prison when she is sentenced. Maher, who retired at age 50 in October 2003, said he faces up to 55 years. Walsh is testifying without a cooperation agreement; Maher said he has one. Both hope Judge Victor Marrero will give them a break when he eventually sentences them.