The issue may be decided for you depending on Irma's track, which no one can predict with 100% certainty.
If it does enter Florida and Orlando is put under a hurricane warning, WDW will close. Unfortunately, your Saturday departure would be right around the time that decision would be made. The other factor to consider is MCO would likely close if Irma is approaching. If your plan was to drive, evacuation routing might make it impossible to even get to Disney.
Again, all of those scenarios are predicated on Irma entering Florida and heading north through it. If the storm instead tracks west into the gulf or east into the Atlantic, it's business as usual in Orlando.
The problem for you is it won't be clear which way things are going until Saturday at the earliest and possibly even later. So you are in a tough position given your scheduled arrival date. Again, if you are flying, I suspect the airlines might start cancelling flights to or from MCO on Friday if the indications at that time are clearly indicating Irma is about to enter Florida. Put another way, that's likely the earliest point where a decision could possibly be made for you.