The 17-year-old started the Championship game against Cardiff City at the City Ground on Saturday afternoon on the substitutes’ bench, but he came on in place of 18-year-old forward Tyler Walker in the 80th minute.
McDonagh was overjoyed to have made his first appearance in a competitive game for Forest, but he was disappointed with the Reds’ 2-1 defeat.
Words can't describe how happy I am, it's a dream come true to make my debut but disappointed about the result #NFFC #— Gerry McDonagh (@GerryMcDonagh64) August 29, 2015
Visiting Cardiff took the lead in Nottingham midway through the first half when Kenwyne Jones rose highest to meet Peter Whittingham's lofted ball.
Joe Mason doubled the Welsh outfit’s advantage four minutes into the second half by diverting Anthony Pilkington's shot with his head.
Michail Antonio scored from a long ball for Forest in the 86th minute, but Cardiff held on for the three points.
Over the course of the 90 minutes, hosts Forest had 58% of the possession, took 17 shots of which five were on target, and earned 11 corners.
Visiting Cardiff had 42% of the possession, took 13 shots of which five were on target, and earned two corners.