So sad to see that happen to such an iconic landmark. We were last there on St Patrick's day, 2013. Hopefully, when it's daylight there, they can pinpoint the cause, and see what's salvageable. It will be rebuilt, but not with wood this time.
I know a Fire Inspector in NYC who told me that they call the roof of St Patrick's Cathedral, the Lumber Yard. That was just remodeled too, and they added a water misting system on the wood ceiling.