If Wikipedia can be considered a reliable source, the Cornish language revival movement has been succeeding, but with a lot of disagreement:
The revival of Cornish began in 1904 when Henry Jenner, a Celtic language enthusiast, published his book Handbook of the Cornish Language. Jenner's work was based on Cornish as it was spoken in the 18th century, although his pupil Robert Morton Nance later steered the revival to the style of the 16th century, before the language became more heavily influenced by English. This set the tone for the next few decades; as the revival gained pace, learners of the language disagreed on which style of Cornish to use, and a number of competing orthographies were in use by the end of the century.
According to the site Teaching English,
Even though the language was described as ‘dead’ towards the end of the 20th century, its revival has resulted in 2,000 people now being fluent in it (as per a survey in 2008).
Eight years earlier, the number of speakers was estimated to be around 300.