I don't expect much of Irabu the rest of the way, so I'd hold on to the closers you have.
Millwood, on the other hand, would make a great pick-up. He's looked very sharp lately, showing very nice control even under pressure. You're not risking very much by dropping Mendoza - go ahead and grab Millwood. If he doesn't pan out, no harm done; if he does, that's the sort of acquisition that can give a fantasy team a huge boost.