Actually, archaeological evidence of Celtic human sacrifice is pretty much non-existent. If it happened at all, it was a "once in a century" sort of thing. Phoenician/Punic evidence is a little more ambiguous, but since Punic inscriptions mention that they happened I think we can take their word for it--but again it wasn't nearly on the scale that 1) the Romans described them (according to the Romans,
everyone but them performed massive human sacrifices

) or 2) the Aztecs performed them (as far as I'm aware, Aztec human sacrifice is just plainly unparalleled anywhere else in the world--even in other Mesoamerican cultures).
Also I think scholars are in pretty much unanimous agreement that the "wicker man" specifically was an invention of Julius Caesar's overactive imagination.

If the Celt's
did perform human sacrifice, it probably involved slitting the throat and dumping the body in a bog (but bog bodies could also represent executed criminals, so...).