The French region of Brittany (which they call Bretagne) is the home of the ancestors of the Celtic people who originally lived in what is now England. They speak Breton, a Celtic language that they brought with them when they fled from England, seeing both the Germanic tribes AND the Norse tribes (who also did, and still do, speak Germanic languages: Danish, Swedish, Norwegian and Icelandic) coming to conquer their original homeland.
They've lived in what is now northwestern France for more than a millennium, and they seem to get along just fine with their neighbors. Obviously, it's a tremendous advantage to speak and understand French, the country's national language, so lots of the Breton people are bilingual.
Just remember that the British Isles and all of their nearby islands (not just Ireland) were colonized by Celtic tribes, taking over from the Stone Age people who built Stonehenge. Those folks never developed a written language. English probably has a handful of ancient words inherited from them, but after so many millennia and the inevitable sound shifts, it's impossible to track them down.
As to your question about other Celtic peoples, there are a few, but they're all within boating distance of England, Scotland and Ireland. Let's see: there's Cornish--which is hardly spoken anymore--and Manx--which is spoken only slightly more than Cornish. And of course let's not forget Welsh. Virtually all Welsh people speak English, and many don't even bother to learn their ancestral language. Nonetheless, the government makes sure that Welsh continues to be a living language, and there are enough Welshmen who feel the same way, so it's not in any danger of extinction.