The round tower at St. Canice's Cathedral is the oldest surviving structure in Kilkenny city, one of 65 towers of similar construction distributed about the island.
These tall, needle-thin stone constructions are of excellent workmanship obvious by their mere survival.
Oddly, there is no record of a Viking attack upon the Kilkenny monastery.
The tower is about a thousand years old on the site of a monastery founded about 1,500 years ago. It's in great condition, although the conical roof is missing apparently removed in a remodelling. Additionally, small gaps and out-cropping stones on the inner surface in a clockwise upward spiral are all that remain of a stone stairway that used to be attached to the inner surface. This stairway was removed in recent years except for the top four steps, in concern for public safety.
The tower seems to have a very slight tilt, but is almost immaculate overall.
A hundred feet tall, the tower's diameter tapers from 15 feet at base, where the limestone walls are about 3 feet thick, to 11 feet at the top, where the wall thickness is about 2 ½ feet.
Tall, thin, and heavy. Excavations in the 1800's revealed that the foundation is only 2 feet deep. Two feet, and set into graveyard clay.
There are in fact graves underneath. These, with folks buried heads-eastward in the Christian fashion, are part of the evidence for earlier monastic activity.
The purpose of the round towers is a matter of question. Irish folk wisdom attributes their function to defense against Viking marauders; and indeed the Norse invasions of Ireland (the first in AD 794) were contemporaneous with construction of the round towers. Indeed, monasteries were the repositories of great wealth treasures, food, and people (slaves/hostages) and primary targets for Viking plunder.
But the chimney-like aspect of the Irish round tower would seem to make it a poor refuge against any attack by fire. Its traditional wooden door, however high above ground, wouldn't be much defense against a flaming arrow, and even if not breached could fuel a mass death by suffocation and for sacred texts a smoke-damage destruction.
Principally, most likely, the Irish round tower served as a belfry, used in the keeping of time for each day's holy regimen. Indeed, linguistic evidence backs up this likelihood, the Irish word for the towers being cloigteach literally, "bell-house." Belfry.