Nobody knows anything about “Celts.” I live in Scotland, a ‘Celtic’ country. Hell, I play ‘Celtic music,’ which is really Irish/Scottish dance tunes that can be actually dated back to the 18th century, at most. Many are far, far more modern than that and have composers who are still alive and well. The most ‘Celtic’ of bagpipes, on the soundtrack of everything labeled Celtic, the uilleann pipes (which I play), were invented in the mid-to-late 18th century.
They’ve found a lot of cool Iron age artefacts in Ireland and Scotland, but people know jack sh1t about Iron age ‘Celtic’ culture. Some of the earliest written texts in Irish are translations of the Bible, and I am well aware of sagas like the Ulster Cycle and so on, but that stuff has all been translated in the early modern era, and it doesn’t exactly tell you how people learned horsemanship. Given you needed horses to move yourself and your sh1t from A to B or deal with some other fella’s army, the answer is most likely, however you could.
But we know so little about the Iron/Bronze Age cultures of Ireland and the UK that we can mythologise to our heart’s content.