Now, authorities say, O'Neill, 23, has become a fugitive. He's been on the run from probation officials since testing positive for drugs and alcohol, thus violating the terms of his parole on the drunken-driving charge, they said.
"Sean O'Neill has been a thorn in the side of law enforcement in Chester County for years now," Hogan said.
"Were we surprised when he tested positive and then became a fugitive? No. Are we happy about it? Absolutely not, and if he's in the United States, we'll find him."
O'Neill's lawyer in the DUI case, Vincent DiFabio, said he was surprised by the turn of events. O'Neill had been attempting to turn his life around, enrolling in classes at Temple University, DiFabio said.
"I thought he was on a pretty positive track after the last court proceedings," DiFabio said. He said he had not heard from O'Neill's family on the matter.
Hogan said O'Neill may have left the country. His father, Sean Sr., was deported to Ireland after police found silencers while searching his Willistown Township home after the 2006 death of 17-year-old Scott Sheridan, his son's friend.
Owning silencers without a special license is a federal firearms violation, and the elder O'Neill served 18 months in jail before being deported.
"We are following trails in the United States and internationally," Hogan said. "If I knew where [Sean O'Neill] was, he'd be locked up by now."
O'Neill's sister Roisin has had an encounter with the law. In 2008, while intoxicated, she drove the wrong way on the Blue Route, striking a car head-on and killing Patricia Murphy Waggoner of Brimfield, Mass., a 63-year-old grandmother.
After his release from a residential treatment program in 2009, Sean O'Neill was arrested again in 2011 when he drove his Cadillac into a home's garage in West Goshen.
His blood alcohol content registered at 0.25, Hogan said, triple the legal definition of drunken driving.
"We're like, here we go with Sean O'Neill again," Hogan said.
Following a stern lecture from a judge, O'Neill was sentenced to two to six months in prison. He was out on parole when he tested positive for alcohol and illegal drugs this month.
Officials then lost contact with the SCRAM alcohol monitor he was required to wear, Hogan said.
"Then they really started amping up the search for him," he said.
Hogan said he had authorized police jurisdictions within the continental United States to extradite O'Neill if he is caught.
"Quite frankly, if he's left the country, Chester County law enforcement will not shed any tears if he never returns to the United States."
Contact Aubrey Whelan at 610-313-8112 or at awhelan@philly.com, or follow @aubreyjwhelan on Twitter.
