I'm writing something about the Celtic languages of northwestern Europe when the Saxons came to Britain, and described a "continuous but loose-knit group of peoples in France and the British isles". A colleague reminds me of the Celtic peoples of Iberia. But I'm writing about the 6th century or so. Were there still Celtic-speakers in Iberia by then? Or is it fair that I describe the remaining Celts in that part of the world as confined to France and the British Isles?
Actually, there are good chances that a Celtic language was spoken in the 6th century in the Iberian Peninsula, not the ones you would expect but a Brittonic one.
We know for sure that a group of Britons settled in Galicia in the 6th century, when Galicia was the independent kingdom of the Suevi: a bishop of the Britons was present at the 2nd council of Braga held by king Miro in 572, and his name was the Celtic [Mailoc]:
Martinus Bracarensis metropolitanae ecclesiae episcopus his gestis Subscripsi.
Remisol Besensis ecclesiae episcopus his gestis subscripsi.
Lucetius Conimbrensis ecclesiae episcopus his gestis subscripsi.
Adoric Egestanae ecclesiae episcopus his gestis subscripsi.
Sardinarius Lamicensis ecclesiae episcopus his gestis subscripsi.
Viator Magnetensis ecclesiae episcopus his gestis subscripsi.
Item ex synodo Lucensi.
Nitigisius Lucensis metropolitanae ecclesiae episcopus his gestis subscripsi.
Andreas Iriensis ecclesiae episcopus his gestis subscripsi.
Wittimer Auriensis ecclesiae episcopus his gestis subscripsi.
Anila Tudensis ecclesiae episcopus his gestis subscripsi.
Polemius Asturicensis ecclesiae episcopus his gestis subscripsi.
Mailoc Britonensis ecclesiae episcopus his gestis subscripsi.
Another contemporary document, the Parochiale Suevorum -an ecclesiastical/administrative division of the kingdom of Galicia preserved just in interpolated later 11th-12th c. copies-, states that the authority of the bishop of the Britons comprehended their own churches in Galicia and Asturias and the monastery Maximi:
Ad sedem Britonorum ecclesias que sunt intro Britones+ una cum monasterio Maximi et que in Asturiis sunt. (download link)
So this Briton community was an ethnic community with churches in Galicia and Asturias and under the ecclesiastical direction of a bishop-abbot.
The Britons kept sending bishops to the Visigothic councils of the 7th centuries, but the See was discontinued either in the 8th century, after the Arab invasion, or either in 830 due to a Viking raid. Today there are several hamlets called Bretoña in Galicia, and one called Bretios < Bretonos, but the town of Bretoña, in Pastoriza, near the city of Lugo, is thought to the be continuation of the old “capital” of the local Britons.
Further references at Wikipedia.
Now, for the native Celtic tongues, they probably became extinct during the fist centuries of our era (I don't have any positive evidence otherwise); but, some considerations:
In his 5th century Chronicle, bishop Hydatius, a native Galician, apparently distinguished among Romans, living for example in the city of Lugo; native Galicians living in rural areas and even in hillforts; and the newcomers, the Germanic Sueves, Vandals, and Goths. Now, since Galician have been Roman citizens for centuries, I guess that this mean that they preserved enough social and cultural specificity as to be accounted as a distinct subject (versus Romans), and in fact they apparently acted autonomously during their wars with the Sueves; but, well, Rome was falling apart.
In the late 7th century Valerio of Bierzo wrote about his life in the Bierzo region, next to Galicia. One of the few uninhabited places he mentioned was called Ebronanto, which is very probably a local evolution of *Eburonantu- 'yew-valley/stream'. This was not a mountain, a river, or an inhabited place, but a pagus, so its name was maybe not too old (assuming that uninhabited places change names much frequently than other places). He also records him and his companions destroying a pagan altar in the top of one hill and building a church in its place.
Local Latin charters written during the high middle ages show the preservation of dozens of pre-Latin words in Galicia, Asturias, Northern Portugal, Cantabria and northern Castille. These words were mostly related to farming and land description (camba 'high valley', busto 'dairy', senara 'farmland', laria 'plot', cotto 'hill', lagena 'flagstone', gandara 'badland', brania 'moor', vaica 'river bank', combarro 'shed', corrego 'ravine', arrogio 'stream', vereda 'road', combona 'tidal trap for fishes'... ) and tree and plant names (carbalio 'oak tree', bitula 'birch tree', amenal 'copse of alder trees', togio 'gorse'...). Further readings and references in Josep Coromines' Diccionario Crítico Etimológico Castellano e Hispánico and in the Léxico Hispanico Primitivo, by several authors.
For the 6th century CE, the answer is a definite no, there weren't any speakers of Celtic languages on the iberian peninsula.
The attested inscriptions in Celtiberic and Gallacean end in the 1st century BCE. There might be some speakers around after the last inscriptions, but six centuries later is a rather long time.
I was in Galicia in 1999. I read that the last speaker of a Celtic language died around the 1600's. I believe he lived in the Ourense area. Galicia and Northern Portugal were one in the same until about 900. There are tens of words in Gallego and Portuguese that are of Celtic origin. Obviously, vulgar Latin came to become the main language under Roman rule, so by 200 AD most inhabitants were speaking that form of Latin, but that does not mean many Celtic words died out or were no longer used. I suggest that the local people of ancient Gallaecia were probably code switching and creating an inter-language for many centuries after Roman control through to the time of the Germanic invasions, much like Spanglish in the USA, a mix of Spanish and English that many Mexican-Americans speak. Eventually the Latin root words and later the Arabic influences from the invasion of the Moors probably leveled this out, though Muslim control in the northwest of the Iberian Peninsula was only for a few centuries, and never did they control Austurias.