Complex sentence: Their kingdom was known as Brigantia and it was centered in what is now known as Yorkshire, the Brigantes were the only Celtic tribe to have a presence in England and Ireland; in the latter of which they could be found around Wexford, Kilkenny and Waterford.
Split into simpler sentences: Their kingdom was known as Brigantia, and it was centred in what is now known as Yorkshire. The Brigantes were the only Celtic tribe to have a presence in both England and Ireland, in the latter of which they could be found around Wexford, Kilkenny and Waterford.