One behind on 72 overnight, the 15-year-old from Forrest Little had seven birdies for a six under par 138 total and claimed the coveted trophy by three shots from Austria's Emma Spitz.
Spits, a joint first round leader, had a 70 and third place went to England's Samantha Fuller (Roehampton), who repeated her opening 71.
It was just two years ago that McCarthy chose golf over football as her sporting passion and she proved it was a good decision with a win in last year¹s Irish Girls¹ Championship.
"I played football for Lienster but than took up golf," explained the youngster. "I've still got another four years to do at school but, eventually, I would like to turn professional.
I shot 68 in the second round and finished fourth here at Strathmore last year so it is definitely one of my favourite courses. Today I didn¹t hole any huge putts but made a lot from around the 15 foot range. This definitely rates as one of my best ever performances."
Introduced to the game by her father, Ronan, McCarthy is coached by Sean O'Grady he is also in charge of the 19-year-old Maguire twins, Lisa and Leona.
"The twins are great and a real inspiration," continued Julie. "I get to meet them quite a lot and we were all on the same National Squad training camp earlier this year."
Aboyne¹s Shannon McWilliam was the leading Scot. She shot rounds of 78 and 75 to finish in a tie for 15th place. Gullane's Charlotte Munro (80,74) and Troon's Hazel MacGarvie (79,75) were another shot back.
The winning team was Ireland.
Photograph: Julie McCarthy and Sheena Scott Lady Captain of Strathmore.