alt Hacker NewsThe Wikipedia article suggest that they could have been erected as far back as 6000-7000 years ago - so older than Stonehenge, and therefor also older than Celtic culture. The Wikipedia article suggest that early Christians defaced and destroyed some of the stones, but knowledge about the people who erected those stones was lost way earlier than 300CE.
I just used Wikipedia as a quick reference in English. There is clear evidence that the practices of ancestors worship and their ancient practices involving these stones was directly linked to Christian persecution, including documented examples of the progroms against the indigenous Europeans starting immediately following crusades and effectively being part of them.