Traditional murder mysteries and solving them are, in the end, people oriented actions. It's not like Irisu didn't know Oreki himself is surrounded by people who might help him.
But then again, perhaps I'm giving Irisu far too much credit. This could have been, realistically, the best way she could think of so it's useless to blame her for not doing things differently. If she had been able of more, it's likely she could have finished the script by herself as well. This was her limit, and people paid the price, not excluding herself. Next time she wouldn't make the same mistakes, just like Oreki wouldn't. A lesson for both of them.