Kimuri people

Japanese: キムリ人 - きむりじん

... Around the 8th and 7th centuries BC, the Celts, who spoke Indo-European languages, arrived, bringing with them the Iron Age culture, and conquered the indigenous peoples and settled there. They were roughly divided into the Goyderians (Gaels), who arrived in the 7th and 5th centuries BC, and the Britons (Cymri), who migrated in the 4th and 2nd centuries BC, from whom the name Britannia (Britain) was derived. They split into many tribes, but their society was dominated by the nobility, made up of priests (Druids) who were in charge of religious ceremonies, and warriors who owned vast tracts of land, who exploited the common peasants. ...

From the Britons

…the Cymri were a Celtic tribe among the Indo-European peoples. They migrated from continental Europe to southern Great Britain in the 4th to 2nd centuries BCE, and established numerous small tribal kingdoms.

*Some of the terminology that refers to "Kimuri people" is listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

Japanese:

…　前8，前7世紀ころからインド･ヨーロッパ語族の言語を使用するケルト人が渡来し，鉄器文化をもたらし，先住民族を征服して住みついた。彼らは大別して前7～前5世紀に来たゴイデル人(ゲール人)と，前4～前2世紀に移り住んだブリトン人(キムリ人)から成り，後者からブリタニア(ブリテン)の名称が由来した。彼らは多数の部族に分立したが，その社会は神事をつかさどる聖職者(ドルイド)と広大な土地を所有する戦士が貴族として支配階級を構成し，一般の農民を収奪した。…