The cester, caster and all variations thereof in place names all come from the Latin for a military fort, which shows you how old these place names are.
Yep. My home town is a caster, and had a cavalry garrison named after this bloke
who lived there from time to time.
Oh, and at least one (maybe more) of my great-grandparents was Irish, and I have an Irish surname. But I don't feel Irish in the slightest. Never even been there or owt.