The Celtic languages today probably have an ancestor language, just like how Latin is the ancestor for Italian, Spanish, etc. So yeah, there probably is an Ancient Celtic "mother language," but it is probably older than Latin. In
Julius Caesar's De Bello Gallico, he makes a point to note that the languages of the Celtic peoples in both Gaul and Britain all speak different languages.
Boudicca was not Irish. She was from the Iceni tribe which had its territory in southeast England. It also is important to note that yes, she was alive during "ancient" times (she was alive at the same time as the
Roman Empire during the reign of
Claudius), but if there is an "ancient" Celtic language, it is probably far older than her.