The Reds were beaten 5-3 on penalties by holders County Antrim.
The Reds beat Co. Armagh 2-0 on Thursday to reach Friday's final.
Four second half goals as United take the lead but suffer from tired legs to Scottish opposition.
Man Utd are in a group with Rangers, Club NI and Co. Londonderry.
Man Utd were well-beaten by Chivas in the SuperCupNI Globe Final.