McCarthy moves into the top eight in the tour’s order of merit having opened the tournament with scores of 67 70.
He would have got himself into a play-off for the title but for two bogeys – his only errors of the day – in his final four holes.
Boston West’s Billy Spooner returned a final-round 68 to be crowned champion on 15 under, his victory push bolstered by a second-round 64 that had followed 69 on day one.
Alex Lodge (Azego: The Component People) pushed him hard on the final day, but 20-year-old Spooner repulsed his challenge to capture his maiden title by a shot.
Spooner had set a new course record with his 64, but it lasted only 24 hours as Ireland’s Brian Casey shot a nine-under par 63 to finish tied with McCarthy after a round that contained nine birdies and one eagle.