Gallagher, 43, will not have to serve any time if he resigns from his council post, effective April 18, and completes an alcohol treatment program.
He apologized to his victim for the July 8 incident.
"While I was intoxicated, I intentionally and forcibly touched intimate parts of the complainant and subjected her to sexual contact without her consent," Gallagher said in court. "My conduct was wrong and I apologize."
The victim, who was in court, did not speak but issued a statement that was read aloud.
"I will never recover from the pain, anguish and humiliation I suffered as a result of this man's vicious assault and attack on me," her statement said. "He has scarred me for life and he took away my dignity and self-esteem."
In exchange for his plea, Gallagher will not have to register as a sex offender. He was ordered to stay away from the victim.
Gallagher, elected to the council in 2001, is one of three Republicans in the 51-member body. He lives in the Middle Village section of Queens with his wife and two sons.
The plea deal was reached after an indictment accusing the councilman of rape and assault was dismissed in January by a judge who said that prosecutors asked Gallagher improper questions before the grand jury.
The district attorney said at the time that he would present the case to another grand jury, noting that the judge had said there was sufficient evidence presented to support the charges.
The councilman had acknowledged an encounter with his accuser but said it was consensual. His accuser told authorities Gallagher offered to drive her home after they met in the bar, but instead took her to his Queens district office, where he attacked her.
In a statement Monday, his lawyers, Benjamin Brafman and Christopher Renfroe, said, "Mr. Gallagher has always maintained that he did not commit the crime of rape. Today's proceedings, however, embarrassing, should put that false allegation to rest once and for all."
