Michael D. Ahern was in the prisoner’s dock in Dorchester Municipal Court today where he pleaded not guilty to charges of  motor vehicle homicide while operating under the influence of alcohol, operating to endanger, and speeding.  He allegedly struck and killed bicyclist Doan Bui, 63, of Dorchester on Sept. 14.
Michael D. Ahern was in the prisoner’s dock in Dorchester Municipal Court today where he pleaded not guilty to charges of motor vehicle homicide while operating under the influence of alcohol, operating to endanger, and speeding. He allegedly struck and killed bicyclist Doan Bui, 63, of Dorchester on Sept. 14.
Aram Boghosian for The Boston Globe

A 46-year-old Dorchester man who allegedly struck a bicyclist on Morrissey Boulevard last week and then stopped to call for help pleaded not guilty today to a charge of motor vehicle homicide while drunk.

Michael D. Ahern was arraigned in Dorchester Municipal Court on the drunken driving charge along with operating to endanger and speeding. With relatives in the courtroom, Ahern pleaded not guilty to all charges, and bail was set at $25,000 cash.

According to Suffolk Assistant District Attorney Patrick K. Devlin and a State Police report, Ahern was driving his 2011 F150 pickup truck on William T. Morrissey Boulevard at 12:30 a.m. on Sept. 14 when he struck a bicyclist who was riding southbound near Malibu Beach.

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The bicyclist, identified by authorities as Doan Bui, 63, of Dorchester, was pronounced dead at the scene by emergency personnel. The impact threw the victim “a significant distance,’’ Devlin said in court.

Ahern stayed on the scene and called 911 for help on his cellphone, Devlin said.

“That’s the only correct decision he made that night,’’ Devlin said describing Ahern’s actions.

According to Devlin and a State Police report, Ahern acknowledged to State Police troopers that had one drink at a Boston restaurant before the crash. Later at Boston Medical Center where Ahern was taken at his request, his brother, Edward, told troopers his brother had hired an attorney.

Trooper Thomas D. Canning wrote in the report because Ahern had a lawyer, he did not question him about the crash. Instead, he asked for identifying information and watched him as he moved about the hospital room where he was being examined by medical staff.

“I did not ask him about the accident. I detected a strong odor of intoxicating beverage coming from his person,’’ Canning wrote. “I observed his speech to be noticeably slurred and his tongue was thick and pasty… I observed Michael to be noticeably unsteady as he walked…’’

Another trooper reported that after examining Ahern’s 2011 Ford F150 pickup truck he concluded the vehicle was moving at 50 miles an hour when Doan Bui was hit, while the posted speed limit is 30 miles an hour. There was no sign of braking on the roadway, the trooper concluded.

In court today, Ahern’s attorney, Stephen K. Clifford of Quincy, described the case against Ahern as “relatively weak.’’ He said State Police noted that Ahern’s eyes were bloodshot, but they did not report that the airbag deployed, nor that Ahern was wearing eyeglasses at the time the airbag hit his face.

Clifford described his client as a successful businessman with interests in a Quincy bakery, real estate, and as a building contractor who is active in his community.

Clifford also said Ahern was one of the many contractors who went to New York City and volunteered to help recover victims of the 9/11 attack from the destroyed twins towers at the World Trade Center.

According to both the defense and the prosecution, Ahern received a 20-year prison sentence in Suffolk Superior Court in 1989 for an arson conviction, the details of which weren’t immediately available.

Ahern has a multi-page driving history, according to the Registry of Motor Vehicles, which suspended his driver’s license indefinitely as a result of the fatal crash, an RMV spokesman said today.

Ahern is due back in court Nov. 21.