Can you bring strollers and car seats on Blue Star Ferries?

Yes you can take them but you will have to hand carry, including at least some stairs / ledges / gaps. You can leave them in the parking area - they usually have a corner section for luggage and bulky items. They won’t be monitored down there but the likelihood of theft / issues is very very low.

We always took what ever we needed on the ferries

Better if you have bought the cheapest deck seating , as that means you have lots of room around you as compared to the rowed airplane seats that have more limited room

Have only ever taken deck or a cabin ( cabin great as we stuffed all our things in there & when someone was tired they could take themselves off for a nap