[Disclaimer: while I am a linguist, I am not an expert on Celtic languages (ancient or modern), and I cannot vouch for the translations offered below. Most of the information in this article comes from the book The Lost Zodiac of the Druids by Gregory Clouter, and it should be noted that the views in the book are not those entertained by most scholars.]
That the ancient druids practiced astronomy and astrology is beyond doubt. It would be amazing if they did not, since practically all ancient cultures did. But beyond that, their astronomical knowledge is specifically cited by many of the Roman, Greek and Irish authors that describe them; and there are even a few archaeological finds that suggest it.
Primary among these is the Coligny calendar, discovered as little more than a pile of bronze fragments in 1897 — most likely smashed by Roman authorities during the suppression of druidic practice — and painstakingly restored piece by piece. Less than half of the calendar remains, but there is enough to clearly see a beautiful time-keeping system that aligned the sun and moon into a single calendar, and listed dozens of holidays, rituals, celebrations, and the like.
But if Gregory Clouter (The Lost Zodiac of the Druids, 2003) is right, the Gundestrup Cauldron puts the Coligny calendar to shame.