A Sagaponack man, David Osiecki, 65, pleaded guilty on May 26 to one of two arson charges, admitting in a Central Islip courtroom that he had set the April 19, 2014, fire that seriously damaged a $34 million house on Dune Road in Bridgehampton. Its owner, Ziel Feldman, was an acquaintance of Mr. Osiecki’s.Mr. Osiecki, who has been diagnosed with a bipolar condition, is in an in-house psychiatric treatment facility, according to an agreement between his lawyer, William T. Ferris, and Suffolk County District Attorney Thomas Spota. If after 15 months he successfully completes treatment, he will be sentenced to five years’ probation with continued psychiatric monitoring.He is additionally accused of setting fire to a Patchogue bar, the Off Key Tiki Beach Bar on Baker Place, on Nov. 3, 2013, causing extensive damage to that building as well. That charge is in abeyance, according to a spokesman for the district attorney, Robert Clifford. If Mr. Osiecki fails to complete treatment, he can be tried on the Patchogue charge, as well as sentenced to a term in state prison for the arson in Bridgehampton.For a time, he languished in a legal Catch-22. Though twice found by county doctors to be mentally incompetent to stand trial, he could not be treated until he was indicted, but the indictment did not happen until last October. Meanwhile, he spent more five months in the county jail in Riverside.