What act did Bloom make on their arrival at their destination?

At the housesteps of the 4th of the equidifferent uneven numbers, number 7 Eccles street, he inserted his hand mechanically into the back pocket of his trousers to obtain his latchkey.

Was it there?

It was in the corresponding pocket of the trousers which he had worn on the day but one preceding.

Why was he doubly irritated?

Because he had forgotten and because he remembered that he had reminded himself twice not to forget.

What were then the alternatives before the, premeditatedly (respectively) and inadvertently, keyless couple?

To enter or not to enter. To knock or not to knock.