In 1833, the two provinces of Dublin and Cashel were merged. Over the centuries, numerous dioceses were merged, in view of declining membership. The same is true for this diocese where it can be seen that each of the entities listed in the title would have been a diocese in its own right.
It is for this reason that the united diocese has six cathedrals (although the union of Waterford and Lismore predates the Reformation).
^Fryde, E. B.; Greenway, D. E.; Porter, S.; Roy, I. (1986). Handbook of British Chronology (Third Edition, revised ed.). Cambridge: Cambridge University Press. p. 382. ISBN0-521-56350-X.