Imagination

Joyce imagines Leopold Bloom and invents his birth and where he was born. This imagining is so powerful, so vivid, so strong that it becomes indistinguishable from events that really happened. And the real building, which was there before Joyce ever imagined Leopold Bloom, the real building in which an imaginary person was supposed to be born on an imagined date after many years gets the above marker.

This may seem strange until one remembers that Bloomsday is a holiday, the only one that I know of, that commemorates entirely fictional events, including those of the person born in the house with the marker.