Celtic Language Resources
The Celtic languages are a branch of the Indo-European language family, descended from the Proto-Celtic language.
During the first millennium BC, Celtic languages were spoken across much of Europe and central Anatolia. Today, they are restricted to the northwestern fringe of Europe and a few diaspora communities. There are six living languages: the four continuously living languages Breton, Irish, Scottish Gaelic and Welsh, and the two revived languages Cornish and Manx. All are minority languages in their respective countries, though there are continuing efforts at revitalisation. Welsh is an official language in Wales and Irish is an official language across the island of Ireland.
Irish, Manx and Scottish Gaelic form the Goidelic languages, while Welsh, Cornish and Breton are Brittonic. All of these are Insular Celtic languages, since Breton, the only living Celtic language spoken in continental Europe, is descended from the language of settlers from Britain. There are a number of extinct but attested continental Celtic languages, such as Celtiberian, Galatian and Gaulish. Gaulish is more closely related to Insular Celtic than either of these two are to Celtiberian; together, Gaulish and Insular Celtic form the Nuclear Celtic subfamily. Beyond that, there is no agreement on the subdivisions of the Celtic language family. They may be divided into P-Celtic and Q-Celtic.
The Celtic languages have a rich literary tradition. The earliest specimens of written Celtic are Lepontic inscriptions from the 6th century BC in the Alps. Early Continental inscriptions used Italic and Paleohispanic scripts. Between the 4th and 8th centuries, Irish and Pictish were occasionally written in an original script, Ogham, but Latin script came to be used for all Celtic languages. Welsh has had a continuous literary tradition from the 6th century AD.
Below are some resources for learning the languages. If you are living in one of the areas that a Celtic language is native to. Learn it! That is the only way to keep it alive.
Start here for Scots Gaidhlig

Start here for Cornish

Start here for Manx
https://gratisglobal.com/learn-irish-free/ - for starting Irish

Start here to learn Welsh

Start here for learning Breton

Scots is not a Celtic language - but here is the link anyway