While baroclinic influences have been underway, these are not the only, even primary, reason for Irene's relatively very low pressure. Recall that Ike as a Cat 2 also had very low pressure (also about 950mb, in fact). Extra large and powerful hurricanes do not always end up concentrating their force within the core, and such has been the case with Irene, in many ways much as it had been with Ike.

This lower pressure is helping result in higher-than-usual surge for the given Saffir-Simpson category. It is important to note that the Saffir-Simpson scale is a wind scale, only. The NHC has dropped suggesting any relation to surge, pressure, anything else, other than wind. However, if one wants to relate the current overall intensity of Irene to the old SS scale, 950mb would have lined up with Category 3.