After a wedding in Barcelona on September 1, we will explore northern Spain for 9 days. Is that sufficient time for travel between Barce, Bilbao and San Sebastian? Would a rail pass be worthwhile? Appreciate your insights.

You can easily fly from Barcelona to Bilbao and nine days is more than enough time to enjoy the area. You can take buses between towns. It is about one hour between Bilbao and San Sebastian/Donostia.

NO railpass! There are wonderful cheap buses. I think it's more like 2 hours between SS and Bilbao, but bus is the way to go.

As suggested : fly Vueling to Bilbao, take a bus to SS after a few days ,fly back to Barcelona from SS.

No railpass - you'd have to be taking a zillion longer trains to even consider it and renting a car would be nice. There are nearly no direct trains Barcelona to San Sebastian and trains take 6-7 hours (one direct tran takes 5 h 40 m - but if wanting to break your journey somewhere maybe consider train -check Renfe for fares and schedules - www.seat61.com - BETS-European Rail Experts and www.ricksteves.com for lots of info on trains - you may want to explore a little around Barcelona by train like to Montserrat, famous pilgrimage mecca about an hour out of Barcelona, on a mountain top. Trains go right there + cable car.

I checked the average bus ride from San Sebastian/Donostia to Bilbao is 1 hr 20 minutes.

why not just head northwards up the coast from BCN? There is loads to explore.

See if you can fly into BCN and home from Bilbao. The 9 days includes Barcelona, correct? I'd want at least 5 days there. The rest between SS and Bilbao, with maybe a night in Getaria.
Option of staying in Catalunya makes sense, too, with that amount of time. Girona, coast, etc.