Gaelic Language is divided into two major groups
Goidelic - Irish (Gaeilge), Scottish Gaelic (Gàidhlig), and Manx (Gaelg).
&
Brythonic - Welsh, Cornish, Breton

All the above are Celtic languages that migrated at different times to what is now Ireland and England.

I am just starting my Scottish Gaelic lessons and having a ball. Just now starting to see some of the similarities.
hope this helps