There are 3 ways to scroll horizontally on an IR.
#1: Scroll down to the bottom of the IR (often past 50 or even a 100 rows) to find the horizontal scrollbar. Drag the scrollbar right. This is annoying as often the most important data is on the top. So you have to scroll back up.
#2: Hold the shift-key on your keyboard and roll your mouse's scroll roll down. Many users don't know about this technique. I know it works in chrome. I don't think it works in firefox? The scrolling is slow.
#3: Click the mousewheel inside of the “data” section of the IR. and move your mouse to the right. This is the easiest technique, but just like #2 many users don't know about this.
My users are elderly or not tech savvy. They don't know about the scroll wheel (for option 3 above) and frankly won't discover it on their own. I have too many users to tell each and every one of them how to do this basic technique. I shouldn't have to teach users how to do this in the first place.
Preferred Solution (Optional)
At the bottom of what's visible on the screen, there should be a horizontal scrollbar that's floating there. If the user scrolls the IR into view this horizontal scollbar will show up. If the user scrolls below the IR, the horizontal scrollbar won't continue to follow the user. It should only appear when the user can see at least 5 rows of the IR… and the bottom of the IR is cut-off (not in view) because it's below the user's current viewport.