"Gan Teanga, Gan Tir"
This Irish Gaelic proverb means literally "Without language, without country" - that is, a people without a language have no country. There are only a handful of the old Celtic languages extant, and many are rapidly losing native speakers, especially in rural areas.
In Eire (the Republic of Ireland), Irish Gaelic has been an official language since independence, but the language continues to decline in primary native speakers in the Gaeltacht (Gaelic speaking areas), although there has been some rise in numbers of people who have acquired it as a second language, and a rise in the number of Gaelic schools which teach all subjects through the medium of Gaelic.
(Note: Irish Gaelic is referred to as "Gaeilge" in the Gaelic language, but when speaking of it in English, the tendency is to use the English language word "Irish" even among native speakers. This was possibly to differentiate it from Scottish Gaelic, which in English is usually called "Gaelic" [though in times past, it was sometimes called "Erse" or "Irish") and in Gaelic is known as "Gaidhlig.")
In Scotland, where Scottish Gaelic only recently obtained some quasi-official recognition, the situation is much the same, as it is in Nova Scotia, Cape Breton, and elsewhere in Canada where it was once spoken widely. Cape Breton in particular was once a bastion of Gaelic, although both have movements which aim to restore the language.
In the Isle of Man, the last native speakers of "Gaelg" (aka "Gailck" i.e., Manx Gaelic) died out, although there is a movement there as well hoping to restore the language.
Wales has had better success with its native "Cymraeg" in some ways, having had a strong language movement prior to gaining quasi-official recognition.
Cornwall, whose native Celtic language is known as Kernewek (aka Kernowek) has a small population of speakers, having brought that language back from the brink. Kernewek is related to Welsh and even more closely to Breton, since there had been a sizable migration from Cornwall to Brittany durning the Saxon Wars. (c. 6th century AD)
Breton, known in its own tongue as "Brezhoneg" is the language of Lower Brittany. and also remains a minority language, with no support or even recognition from the French government. (See the International Committee for the Defense of the Breton Language site for details.)
On-Line Resources for Irish and Scottish Gaelic
alphaDictionary - Irish Dictionary / Grammar
Dictionary and Language Library
eDIL Electronic Dictionary of the Irish Language
Foclóra agus liostaí focal Gaeilge ar an Idirlíon
Dwelly Online Gaelic Dictionary
English index to Gàidhlig air an Lìon
Scottish Gaelic language, alphabet and pronunciation
Spell checking for Scots Gaelic
Internet Archive Search: Gaelic
Early Gaelic Book Collections - Digital Archive - National Library of Scotland
DASG - Digital Archive of Scottish Gaelic
Broad Scots (aka Doric / Lallans) Online