Manresa and Montserrat in a Day

Yes, it is possible. I probably would do Barcelona > Montserrat > Manresa > Barcelona

You can very easily DIY, which is always more alluring and adventurous. You want to take the R5 train line from FGC at Plaça Espanya in Barcelona to either Monistrol de Montserrat or to Olesa de Montserrat (depending which method you've chosen to ascend to the monastery). Once you're done, at the same train station you can take the train to Manresa, which is under 30' away. Easy peasy. Check schedules for the line here: https://www.fgc.cat/en/ -btw, the line is "Llobregat-Anòia"

For Montserrat check my post: https://community.ricksteves.com/travel-forum/spain/a-visit-to-montserrat-the-holy-mountain-near-barcelona-in-catalonia

Regarding Manresa, I would imagine you've been attracted by the Cova de Sant Ignasi, right? Note though that Manresa is much more: http://www.manresaturisme.cat/ang

In any case, I recommend an early start and planning for a full day in all.

Enjoy!

Thank you so much for your response - it is very helpful! How much time would you allow for the visit to Manresa? We would probably do the guided tour of the sanctuary/cave and would like to see a little more of the town. I assume the town is fairly walk-able? Is the train station close to the cave?

Yes, about 200 yards -albeit a tad uphill :)

Yes, as in most European city, the part you'll want to walk is the historical centre, which is at 200 yards from the station. It's very walkable.

I would probably visit first the Cova and then walk the city centre and see other things (see link above)

Note that being a "country-side area" -so to speak- most things in the city (stores and attractions) will be closed from 2ish to 5ish, so make sure you check your schedules in advance.

Btw, your station should be Manresa Viladordís, which is closer to the cave. Once you finish, if you decide to walk the centre, you can catch the train back to Barcelona in Manresa Baixador, which is closer to the centre.