Not necessarily. Remember when Lambo came back from the future in the Rings Arc and had no memory of actually fighting against Levi in a battle? The theory that perhaps he was from just one of many possible alternate futures was brought up. Then, later in the fight after 20 years later old Lambo showed up, 25 year old Lambo obviously remembered the fight, because he looked at the horns that were written on and commented "Ah, what Gokudera wrote back then".
What this means is that can apparently swap between different futures at any given time. After all, Lambo of 15 years old didn't have a memory of his fight, but the ten year later version summoned by him did. It probably has the same chance of working in reverse. In this case, Gokudera was shot, so maybe he was sent back to the current time line and killed Irie Shouichi. However, judging by the above, an event that happens in the past doesn't necessarily affect a being of the future, and, going even further, an even that happens in one part of the past may not have an effect on one version of the future, but it may further affect an even more distant alternate future.
Gokudera and co. may have killed Irie, but that doesn't mean that the future affected was the same one that Tsuna and the others went to. Pure conjecture, but it kind of matches up with the info of time travel Amano's given us.