Al you can do is vacuum with a brush attachment.. and use a stiffer brush to get down in crevices.
I would use a Shop Vac and vacuum it carefully. Then wipe it down with old rags.
Use canned air (same thing used to clean electronics) to blow it out of the cracks, then vacuum
Are you speaking of the facade of the fireplace or the interior fire box? Two very different cleaning jobs. the facade can be cleaned in the way others have suggested, the second, involves the floor of firebox, the walls and the roof of the firebox.