Because that's how DNA works!
That's still a very high percentage of Irish. I can trace 4 great-grandparents and their ancestors back to Ireland and I'm still only 47% Irish. Your DNA is passed down from generation to generation, that of course includes the generations that you probably don't know much about going back several hundred years.
Since your half sister only shares half of your DNA it makes sense that you're not exactly the same. Even if she was your full sister she still could have a different ethnicity makeup than you, as there's no exact formula for who gets what. My mom had a DNA test done shortly after I did and she only got 20% Irish, even though the 4 great-grandparents I traced to Ireland were her grandparents. It's really just a random mix! I also have a small percentage of North African that I have no idea where it comes from. I think it's kinda cool. :)