Learn Portuguese (Portugal): A ironia é óbvia, por isso ele espera um riso. - The irony is obvious, so he expects a laugh.

This sentence uses por isso (so / therefore), which connects two statements of fact and takes the indicative mood (espera). The concessive structure you are learning means "however [adjective]" and requires the subjunctive. If we changed the sentence to use that structure, it would look like this: por óbvia que a ironia seja, ele não riu (however obvious the irony may be, he didn't laugh).

How do we know espera means "expects" here instead of "waits" or "hopes"?

The verb esperar covers all three English meanings, so you must rely on context. If it meant "waits for", you would typically see the preposition por after the verb, forming espera por um riso. While "hopes for a laugh" is a valid interpretation, "expects" is chosen in the translation because the first clause establishes the irony as an obvious fact, making a laugh the logical, expected outcome.

What is the difference between um riso and uma risada?

Both mean "a laugh", but um riso is more general and often refers to a standard laugh, a chuckle, or the concept of laughter itself. Uma risada specifically implies a loud, open burst of laughter. Since he is just expecting a standard reaction to an ironic statement, um riso fits perfectly.

Is the pronoun ele strictly necessary after por isso?