There is much debate about whether or not Ennis actually says "I love you." There is now a whole group of newbies who swear he breathes it into the shirts when he finds them...If he doesn't then they don't say "I love you" at all. ) WE all know he does not) I think it's better that way, the audience doesn't need to hear it.
From all the imagery and actions in the film we know that they are deeply in love with each other. The dramatic scene at the end, where they argue and break down exemplifies their passion for each other. So Let address those scene where they say I Love You with their actions an other words....
First example that comes to mind for me is the "NO more Beans" comment from Jack, Ennis who doesn't eat soup orders soup and tells the Basque that he's sick of beans. Sweet! Others?