Continental Gaelic Languages

They're Dead, Jim 

These three Gaelic languages are extinct and have been for a long time. However, there are some folks trying to revive Gaulish in a modern version. Of interest might be the fact that all three of these sound/look a lot like Latin—and also a lot like each other.

A Map of Gaul around the Time of the Gallic Wars via Wikipedia

Gaulish 

The Gauls lived in what is now modern-day France as well as Switzerland, Luxembourg, Belgium, and bits of Northern Italy, the Netherlands, and Germany. Some attempts to revive their language as "Modern Gaulish" are underway.

A page dedicated to the revival of Gaulish and explaining “Modern Gaulish,” which is a version of the language adapted for modern speakers. http://moderngaulish.com/

Memrise. https://www.memrise.com/course/426837/the-gaulish-language/

The Sound of the Gaulish Language—YouTube Video. https://www.youtube.com/watch?v=LPavXAL7o9k

Modern Gaulish. http://moderngaulish.herokuapp.com/

Wikipedia has a nice breakdown of phonetics, morphology, syntax, and other linguisticky thingums. https://en.wikipedia.org/wiki/Gaulish_language#Phonology

A Map of the Distribution of Paleohispanic Languages, Including Celtiberian, via Wikipedia

Celtiberian 

Celtiberian was spoken on the Iberian Peninsula, i.e., Spain. Wikipedia has fun Phonology info. https://en.wikipedia.org/wiki/Celtiberian_language#Phonology

The Sound of the Celtiberian Language You Tube Video. https://www.youtube.com/watch?v=zCeHY2T0X3s

Phonology and general classification information. http://www.languagesgulper.com/eng/Celtiberian.html

A Grammatical Compendium of the Celtiberian Languages, from the Journal of Interdisciplinary Celtic Studies. This sucker is 102 pages long. Enjoy! PDF link. https://www4.uwm.edu/celtic/ekeltoi/volumes/vol6/6_17/jordan_6_17.pdf

Cisalpine Gaul (Northern Italy) by © Sémhur / Wikimedia Commons

Lepontic 

The Mostest Extinct of the bunch. It was spoken in the lake region of Northern Italy (see the "Lepontiens" label in the map above, which I think is in French because why not?) between 700 and 400 BCE, though it probably dates back farther than that. It was written in an Etruscan alphabet and is assumed to have absorbed Etruscan influences, though Etruscan has yet to be translated/deciphered. Some information about it:

At Omniglot. https://www.omniglot.com/writing/lepontic.htm

Celtic in Northern Italy: Lepontic and Cisalpine Gaulish. https://rootsofeurope.ku.dk/kalender/arkiv_2012/celtic_spring/Lepontisch_WS_2010.pdf/ PDF Link (only 14 pages).