The film Ossie is watching in the cinema is Back to the Future Part III (1990).
The horse's name was Tír na nÓg, which means "land of eternal youth" in Irish.
Ellen Barkin and Gabriel Byrne were married to each other when the film was made.
Continuity: The photograph of Mary Riley shown at the beginning is larger and more widely framed (showing much of the front of her caravan) than the "same" picture shown at the end (showing only Mary and her baby in the door).
Revealing mistakes: In several shots of Tito and Ossie riding, Ossie is represented by a very floppy dummy that's obviously tied and pinned to the stunt rider playing Tito.
Continuity: The horse ranges in color, from gray with darker gray mane and tail, to almost completely white, and a very noticeable braid in his mane disappears and reappears throughout.
Continuity: The dogs used when officials are tracking the boys and the horse through the forest change from crop-eared to natural floppy-eared.
Crew or equipment visible: When the grandfather is arriving at the Towers in his caravan, another man standing in the caravan is clearly visible, even though the grandfather's only companions were his dog, his black horse and Tir Na nOg.
Continuity: At the cinema, Ossie buys three chocolate bars at the refreshment counter and leaves. Yet while watching the film, he is shown eating popcorn.
References
Shane (1953)
Buono, il brutto, il cattivo, Il (1966)
From Noon Till Three (1976)
Features
Butch Cassidy and the Sundance Kid (1969)
Back to the Future Part III (1990)