Did the celtic pagans of the ancient world practice witchcraft?

Did the celtic pagans of the ancient world practice witchcraft?

Not to your way of thinking.

No, they practiced Paganism.

Sure, at least in Ireland
they put themselves into trances, astral projected to other realms, consulted the spirits of the dead, read the entrails of animals.
used special amulets made out of snakes
made sacrifices to their Gods, spirits and forces of nature and natural formations
they practiced Augury.

St Columcille went to the Picts in what is now Scotland and participated in 3 day spell casting/rune singing challenge with the druid of the Picts. If Columcille won, the Dalriada had the right to settle in Pictland. That was in the 500's though, not really ancient.

Some still do thee things.and apparently it is coming to light that the Irish are not all that Celtic.

The common people probably didn't, or, if they did, they were simple spells. They left the big stuff to the local shaman/druid/high priest/grand pub (whatever he was called).
By simple spells, I mean mostly things that would be considered folklore and/or superstitions. For example, hanging a string of hazelnuts in the child's crib to keep it safe, burning hair and nail filings so a "bad" witch could not use them for a bad spell.

The history doesn't quite go into enough detail to say for sure, but the chances are that they did practice some things that might be considered witchcraft today, like divination or some sort of folk magic.

I would say they were superstitious some tribes believed in protective symbols like Christians believe the cross will protect them the Celts painted protective symbols on their body's to go to battle it also helped scare their enemies.They were just farmers and warriors.Because they had no written languages the druids job was to remember everything and pass it on to new generations.