How do I address my husband's delirium. Should I ask what is he talking about or who are you talking to, or should I just ignore it?

Ignore it. Do what is easiest for you.

Yes, you ignore it. It isn't rational. It can't be dealt with with rational approach.

As long as he is safe and out of harms way, it is okay to ignore it. You are not expected to address every issue especially if he seems content and having a conversation alone or with an imaginary person.

Use this time to catch up on housework, fixing a meal or your favorite TV program.

Now if he is suffering from delirium because of an acute medical condition would require immediate medical care.