Celts and human sacrifice

The Celts practiced human sacrifice on a limited scale as part of their religious rituals. The evidence for this is as follows:

Writings by Romans and Greeks (usually with a point to make) and often at second hand or hearsay
Irish medieval texts
Archaeological data

The most reliable evidence is certainly the lattermost. The Lindow man is the best example: a human sacrifice from the 1st or 2nd century CE, preserved in a peat bog in near perfect condition. The evidence for sacrifice is fairly incontrovertible: he was throttled, clubbed around the head and had his throat slit. This dovetails with the three-fold death detailed in medieval texts.