Do I need to take my online 90-day report confirmation to immigration, or is the process complete?

Ciarán Fagan the online system immediately sends you a confirmation of your submission with the form. That's not an approval, that is the next step after someone looks at it. It will almost certainly send you a rejection at that point. I tried it once after an initial entry and that was what happened, I got the confirmation email and then next day a rejection.

You can do it by post in future, as an alternative to going to immigration, if you have time (more than two weeks). This time you need to go to immigration in person.

It's unlikely it will be accepted. First time in person after each entry.

Your first report after any entry must be in person. The only way you could file online successfully would be if you had an extension in country.