The young Magpies fell behind in their final Premier League Cup Group F fixture at the Impact Arena following Manni Norkett's scrambled close-range finish, minutes after Murphy's glancing header struck the inside of the post.
However, the Galway-born defender levelled the scoring five minutes after the restart, heading home from Amadou Diallo's corner before the latter turned from provider to goalscorer three minutes later, netting from the penalty spot to ultimately secure a first away win of the 2023/24 campaign for United's second-string.
The 19-year-old, who made his Premier League debut for Newcastle in a 4-1 win against Chelsea in November, told nufc.co.uk: "It was an emotional moment for me. I found out my granny passed away yesterday morning so once I heard that, I knew I'd score today. I just had a feeling.
"I hit the post in the first half, which I was gutted about, but I knew I'd score if I got another chance and that goal was for her.
"She was a massive part of my life. I always used to be down at her house when I was growing up and we all loved her so it was a sad loss but I'm going home for her funeral tomorrow so it'll be good to see my family and remember her."
Murphy, who joined Newcastle from Galway United in the summer of 2022, is hopeful the young Magpies can build on their comeback victory against Forest's youngsters when returning to Premier League 2 action on Friday, 9th February ahead of an away clash against Middlesbrough's under-21 side.
The Republic of Ireland under-19 international added: "Coming from behind to win shows the character in the group. At half-time, we knew we were playing well and once we scored the first goal, we knew we'd go on to win the game. There was a feeling about the game so it was pleasing.
"The win should give us a lot of confidence especially with Middlesbrough next, a huge game, and it shows we can win games and hopefully we can pick up a few more going forward.
"It's been a while, especially in this competition and before the game, we said we had to win it to finish on a high note.
"Going one-nil down at half-time was a bit harsh as we didn't play badly and had a few chances but we're delighted to get the win and finish positively."
