Elliott played in Watford full-back, Stephen Kelly, who crossed for Bolton's Johnathan Walters to head in at the back post on 61 minutes.
Walters, making his debut at U21 level, then headed his second on 71 minutes to complete a 2-0 win.
In the first half, Elliott had a shot blocked and fired just wide while he also crossed for O'Connor to hit the bar with a volley.
Glenn Whelan managed to get a brief run-out in the closing stages but Willo Flood's debut for the U21's will have to wait until another day.
Republic of Ireland: Connor; Kelly, Thompson, Fitzgerald, Kohlmann; Thornton, Keegan, O’Connor (Whelan 90), Hoolahan; Elliott, Walters (Deane 81).