The Cork native, who guided the Saffrons to an All-Ireland intermediate title in the 1970s, is a regular visitor to the north coast but this will be his first serious involvement with the county senior team.
O’Mullan, who led Loughgiel to an All-Ireland Club title in 2012, was appointed manager on Wednesday and described McCarthy’s arrival as “a huge coup”.
“Justin has a serious passion for Antrim hurling and it has a special place in his heart. I’ve got to know him over the years because he comes up here all the time.
“He has been there and done that and for me to have a mentor and selector of that stature, it doesn’t get any better than that. We are lucky to have him.”
It’s anticipated McCarthy will make fortnightly trips north. He will attend all of Antrim’s matches next season, with immediate promotion from Division 2A and the Christy Ring their top priorities.
“We have training weekends planned and Justin will be heavily involved in all of that as well as on match day,” said O’Mullan.
“We have a short window – the league starts in February and the Christy Ring is over on the first week in June so those are going to be four or five very important months for Antrim hurling.”