Hirai played near-flawless golf, firing a 3-under-par round of 68 for a two-day total of 141, winning by six shots over first-day leader Billy Wingerd, of Overlea.
"I wasn't thinking about anything," Hirai said. "I was just playing it safe, making no mistakes and having fun out there."
"He told me last night he was going to shoot a 69," Neff said. "I told him it's out there if he stayed focused. He was enjoying it the whole time."
Hirai was spectacular in the final six holes. He birdied the par-5 13th after reaching the green in two shots, then followed up a bogey on No. 14 with birdies on the next three holes to take the title.
"I was doing that all day today," Hirai said. "When I missed the chip, I made the putt. When I bogeyed 14, I told my playing partners I was going to go birdie, birdie, birdie."
Grier shot a 5-over 77 for a two-day total of 158, winning by a stroke.
She started with a 35 on the back nine to take the lead, then hung tough after a triple-bogey on No. 1 left her in a battle with defending champion Calli Vance, of Arundel High School in Anne Arundel County, Md.
"She was 1-up after 11, then I was 1-up," Grier said. "We just kept going back and forth."
"Some people have a bad hole and it's over with," Neff said. "I don't think Ashley thought about it. It was one hole, and she had eight to go."
Grier went up by one on No. 8 when Vance three-putted, then clinched the title with a three-foot par putt on No. 9.
"I was pretty nervous," Grier admitted. "I knew I had to make (that putt) to win.
"I'm glad I won. I had a really good year, and I wanted to end it with this win."