La Rioja, Errioxa in Basque, is modernly a province and autonomous community of Spain and has been held by Castile since the 12th century, excepting minor parts still belonging to Araba. However it was earlier an important part of the Kingdom of Pamplona (later Navarre), which even moved its capital to Nájera (Basque Naiara), where many Pamplonese monarchs are buried.
Previously it was maybe part of the Visigothic marche against Basques known as Duchy of Cantabria, which has left the toponym Sierra de Cantabria and the legend of the destroyed City of Cantabria, maybe Iruña-Veleia in nearby Araba and earlier part of the Roman province of Tarraconensis (earlier Hispania Citerior), a region sometimes known as Ager Vasconicum (the fields of the Basques or Vascones). From this period we know that three tribes inhabited it: Vascones at the East, Berones (believed Celtic) at the center and Autrigones at the West. In the Iron Age it was penetrated indeed by late Urnfield culture offshoots and evidence of violent struggles has been found, most notably maybe in the once prosperous town of La Hoya, whose upper layer is full of bodies slain on the spot, probably by Celtic invaders.
Follows direct translation (my work) from original interview in Basque language at Berria newspaper this Saturday (found via Ostraka Euskalduna):
According to some scholars, Basque language arrived to La Rioja in the 10th century, together with the Kingdom of Pamplona. However, there are also researchers who argue that Basque or proto-Basque was spoken there before Romans arrived. One of them is historian Eduardo Aznar (Barcelona 1977).
What have you gathered in the book "El euskara en La Rioja. Primeros testimonios" (Basque language in La Rioja. Earliest evidence)?
This book is the first part of a work explaining that Basque language had a presence in La Rioja. In these books, I research the oldest Basque indications, and later, the second part, which deals with Middle Ages and modern toponimy, will also be published.
Which are the first evidences that Basque language left in La Rioja?
In the first book, I work with some onomastic materials from funerary slabs that were uncovered in the 1980s. These slabs appeared at the district of Tierras Altas, in modern Soria province (Castile-León, Spain) but they fit in the geography of La Rioja. Of these slabs, so far 11 appear to be indigenous proto-Basque. We believe that they were indigenous people who lived under Roman rule: they look Basque by the nicknames. The most clear example is Sesenco, the slab with that nickname also carries an image of a bull at the base. In the book, besides funerary slabs, I work with ancient toponymy and data from the period, looking for Basque traces.
Nevertheless, some experts defend that Basque language only arrived to La Rioja with the repopulations organized by the kings of Pamplona.
In my opinion, evidence like these slabs do confirm that Basque language was in La Rioja before the kings of Pamplona and before the Romans arrived. After researching these slabs and the toponymy, to say that Basque language arrived only with repopulations is to play with preconceptions.
Some researchers say that at Roman arrival, Calahorra was the main city of the Vascones.
Yes, of course. All classic authors say that Calahorra was a Vasco city. Today a lot of experts try to claim that Kalagorri was Celtiberian but there is no evidence to support that. Another thing would be whether the local language was only proto-Basque or more languages were also spoken. For instance, it has shown up that local coins with the legend Kalagorrikos. To Basque-origin Kalagorri, Celtiberian suffix -kos was added. We may think that, maybe, in spite the Vasco origins of Kalagorri it seems that there was a Celtic elite.
However, if you go to the Roman Museum of Calahorra the word Vasco(-nes) does not show up anywhere. Why?
In my opinion, it is something within the intent of the authors. Sadly, there are a lot preconceptions in this aspect, and all the evidence is against what some researchers claim about Vascones only holding at the Pyrenees. Seemingly, in all this matter scientists do not combine the ideas that actually exist.
In the past, were you defending the early Basque-ness of La Rioja?
There were other authors. But this research is the deepest and most developed so far. Following the books order, first resarch was done by Fray Mateo de Anguiano a Riojan erudite who published in 1704. Already by the end of the 17th century, Anguiano knew that there were many Basque toponyms in La Rioja. However, the one to get deepest in the matter was Basque academic Juan Bautista Merino Urrutia, who researched in the middle 20th century. He was the one to make know, for example, his native Ojacastro town's fazaña or sentence. In those documents from between 1234 and 1239 it is shown how the Supreme Judge of Castile jailed the Mayor of Ojacastro for allowing to make declarations before tribunals in Basque language. Luckily for him, he was later freed, as his stand was allowed by the local old law.
Appendix: Fernando Fernández Palacios, Actualización en onomástica Vasco-Aquitana. Acta Paleohispánica 2009 (PDF).
The introduction is in Spanish but it is essentially an incoplete collection of onomastic and theonymy from the Southern Basque Country and neighbouring areas in Spain, and, curiously enough, a German and a Sardinian site. It includes some of the Riojan slab names. Thanks to Heraus.
 Sesenco: must be little bull < zezen (bull) + -(s)ko (diminutive in Aquitanian epigraphy, now -txo or -txu)
 Vascones, singular Vasco per Wikipedia (properly sourced). From which modern Spanish and others Vasco (Basque). Typically in Spanish vascon (derived modern Basque baskoi) is used but this does not seem to be correct in Latin, only making sense via Romances, where plural is often made by the addition of -es or -s. However Vascon might have been correct in Vulgar Latin I guess, but still I'm sticking to classical Latin grammar while using English.
 See my brief discussion with Heraus on related Aquitanian (Gascon) toponym Calagorris Convenarum at his blog Discover Gascony!
 Definitively the advance of Iron Age Urnfields (influenced by Hallstatt) to the Iberian Plateau was through the Upper Ebro: La Rioja mostly and to some extent also Araba and the lowlands of Navarre. This was a crucial step in the eventual Celtization of the Iberian Plateau and Western lands (Lusitania, Gallaecia).