A Dorchester man was arrested Wednesday in a fatal shooting on Christmas Day 2013, police said. Members of the Boston police Youth Violence Strike Force located and arrested Ian Holness, 27, in the slaying of Rashaan O’Neil, 34, of Mattapan, police said. At about 4 a.m. Dec. 25, police responded to a report of a person shot outside a party at 92 Wales St. in Dorchester, said police spokeswoman Nicole Grant. O’Neil died at the scene, and an unidentified woman was taken to a local hospital after being injured by glass debris, police said. Holness is expected to be arraigned Thursday in Dorchester District Court.Jennifer Smith can be reached at email@example.com.