Nottingham Forest’s Liam Kearney gave the Irish the lead after 29 minutes in the Al Nahyan Stadium in Abu Dhabi.
But goals from Jung To Gook and Kim Dong Hun gave the South Koreans a 2-1 victory.
Another United defender, Joseph Kendrick, was on the bench for the Irish but didn’t get on.
The Irish team also play UAE on Wednesday and France on Friday.