considering the kelts had large farming estates i would be surprised they didn´t had vermin problems i mean where there is grain there´s a rat wating to steal it

but rats can be dealth with falcons dogs foxs and an assort of other animals including snakes so a cat wouldn´t be that much more effective except for one of their hunting tactics wich includes giving the mice a disease wich in turn makes the mice fall in love with the cat urine smell and you get where this is going (so one solution to a disease spreading vermim ended up trading one for another disease spreading animal ) (toxoplasmosis i believe is the name of the disease)