"I was very pleased," O'Hagan told www.UlsterRugby.com. "It was a real experience to play out there and to make my first appearance in front of a such a big crowd (at Kingspan Stadium) was amazing. I wasn't expecting my chance this early but I loved it.
"I was more excited than nervous. It was just such a great experience. The crowd and the intensity of the game were amazing. I came on in the 60th minute and by the 65th I was blowing but it was just incredible."
O'Hagan is not the first member of his family to experience success in the province. His grandfather is Down GAA legend Patsy O'Hagan, who won two All-Ireland SFC titles with the county in 1960 and 1961.
Patsy was also the first Irishman to score a hat-trick at Wembley Stadium. In the early 1960s, a number of Gaelic teams played in a tournament at the world famous London venue and O'Hagan scored four goals in a match against Galway.
"My family are overjoyed that I am playing rugby here. My dad has always been my number one fan and he has really helped me on my way."
Having made his first appearance for the Ulstermen, O'Hagan is now aiming to play as much as possible for his new side.
"We will go over videos and see what we can improve on. Every area needs work and I am just aiming to keep improving and to get better. I am aiming to wear that jersey again. Once you get a feel for it you get the hunger."
Talented, hard-working and proud to play for Ulster. In time Sean could be another O'Hagan to become a sporting legend in the province.