It may have seem that Jack and Ennis at times were battling each other, but they were fighting for the same thing in different ways. Ennis was more of a conformist, his love for Jack had to fall within the parameters of his conformity, but Jack totally went by faith, faith in his love for Ennis, he believed deeply that Ennis would come around understand, and spent most of his life doing just that faithfully believing in Ennis.
Ennis didn't understand Jack's faith until after his death, and inherited it and carried on with that same ritual Jack had. The blood stained shirts were a symbol of Jack's love he left behind, and Ennis's love for Jack continued this faith based ritual.