The Senegalese side ran Paul McGuinness' youngsters close but a solitary second-half strike proved decisive.
The identity of the goalscorer is the subject of much debate with Jack Barmby and Tom Lawrence probably arguing over who should take credit. Barmby, looking to maintain his run of scoring in every game, shot past Tifiane Done and the ball was going over the line when Lawrence appeared to have the final touch.
It was a hard-fought win as a midfield scrap ensued with Jonny Sutherland not overly extended in the United goal as the two sides largely cancelled each other out.
Etoile managed a few efforts and Sidibouyeu Thiandoum shot just wide while the Reds almost profited from one exciting run by James Wilson.
But one goal proved enough and the Reds will face Aspire in Friday's final at 8.15pm at Ballymena Showgrounds after the Qatar team's 6-1 demolition of Desportivo Brasil in the other semi-final.
United: Sutherland, Hendrie, Blackett, Ekangamene (Rudge), McCullough, Rothwell, Daehli, Wilson, van Velzen, Gorre (Barmby), Lawrence (Weir).