Please note that Mommsen uses the AUC chronology (Ab Urbe Condita), i.e. from the founding of the City of Rome. You can use this reference table to have the B.C. dates
The universality and the strength of the Celtic national feeling
would be inexplicable but for the circumstance that, amidst
the greatest political disruption, the Celtic nation had for long
been centralized in respect of religion and even of theology.
The Celtic priesthood or, to use the native name, the corporation
of the Druids, certainly embraced the British islands and all Gaul,
and perhaps also other Celtic countries, in a common religious-
national bond.
It possessed a special head elected by the priests
themselves; special schools, in which its very comprehensive
tradition was transmitted; special privileges, particularly
exemption from taxation and military service, which every clan
respected; annual councils, which were held near Chartres
at the "centre of the Celtic earth"; and above all, a believing people,
who in painful piety and blind obedience to their priests seem
to have been nowise inferior to the Irish of modern times.