The round towers are a unique Irish form of architecture (only three in Scotland and one on the Isle of Man). Though there is no certain agreement as to their purpose, it is thought they were principally bell towers or places of refuge against raiders. Generally found in the vicinity of a church or monastery, the door of the tower faces the west doorway of the church. In this way it has been possible to determine without excavation the approximate site of lost churches, where the tower still exists. Surviving towers range in height from 18 metres to 40 metres, and 12 metres to 18 metres in circumference; that at Kilmacduagh being the highest surviving in Ireland (and leaning 5.3 metres out of perpendicular). The masonry differs according to date, the earliest examples being uncut rubble, while the later ones are of neatly joined stone work. The lower portion is solid masonry with a single door raised two to three metres above, often accessible only by a ladder. Within, in some, are two or more floors (or signs of where such floors existed), usually of wood, and it is thought that there were ladders in between. The windows, which are high up, are slits in the stone. The cap (roof), is of stone, usually conical in shape, although some of the towers are now crowned by a later circle of battlements.

The towers were probably built between the 9th and 12th centuries. In Ireland about 120 examples are thought once to have existed; most are in ruins, while eighteen to twenty are almost perfect. There are four examples outside Ireland. Two in north-eastern Scotland; the Brechin Round Tower and the Abernethy Round Tower.

Famous examples are to be found at Devenish Island, and Glendalough, while that at Clondalkin is the only Round Tower in Ireland to still retain its original cap. County Kildare has the most, with five towers, at Kildare Cathedral (which is 32 metres high), and also at Castledermot, Oughter Ard and Old Kilcullen. The only known round tower with a hexagonal base is at Kinneigh in County Cork, built in 1014.

The purpose of the round towers is a matter of question. A popular hypothesis in the past was that the towers were originally used as places of refuge against raiders such as Vikings. If a lookout posted in the tower spotted a Viking force, the local population (or at least the clerics) would enter, using a ladder which could be raised from within. The towers would be used to store religious relics and other plunderables. However, there are many problems with this theory. Many towers are built in positions which are not ideal to survey the surrounding countryside and would not work efficiently as watch towers for incoming attacks.

The doors to these towers would have been wooden and therefore easily burned down. Due to the almost chimney-like design of the towers, the smoke from the burning door would have been carried upwards inside the tower causing any occupants to suffocate. Indeed, the round towers at Dysert O'Dea and Aghagower show evidence of fire damage around the doorway. There are also records of people being burned to death in round towers.

The main reason for the entrance-way being built above ground level was to maintain the structural integrity of the building rather than for defence. The towers were generally built with very little foundation. The tower at Monasterboice has an underground foundation of only sixty centimetres. Building the door at ground level would weaken the tower. The buildings still stand today because their round shape is gale-resistant and the section of the tower underneath the entrance is packed with soil and stones.

The distance from the ground to the raised doorway is somewhat greater than that from the first floor to the second; thus large, rigid steps would be too large for the door. Excavations in the 1990s, revealing postholes, confirm that wooden steps were built. However, the use of ladders prior to the construction of such steps cannot be ruled out. Therefore, the primary reason for the round tower was to act as a belfry imitating the continental European style of bell tower which was popular at the time. The Irish word for round towers, cloictheach, literally meaning bellhouse indicates this, as noted by George Petrie in 1845.

Daniel O'Connell's tomb at Glasnevin Cemetery had a round tower built above it after his burial in 1847. At Saint Mary's Cemetery in Milford, Massachusetts a round tower was built of Milford granite in the late 19th century as a memorial to central Massachusetts' Irish immigrants, of whom thousands are buried there. Another "revival" round tower was built in 1997 in the Island of Ireland Peace Park in Belgium, as a war memorial to the soldiers of the island of Ireland who died, were wounded or are missing from World War I. The 34 m tower is in the traditional design of an Irish round tower and is partially built with stone from a former army barracks in Tipperary. In 2002 Tony Ryan, a native of Tipperary, built a round tower at his Castleton Lyons Stud in Kentucky.