Standing at the monastery of Dysert Aonghasa (Carrigeen), this exceptional round tower represents one of Ireland's most thoroughly documented and archaeologically investigated examples. Rising to 20 meters in its current truncated state, the tower demonstrates remarkable preservation and sophisticated medieval engineering, while excavations in 1849 revealed fascinating evidence of its construction history and later use.

Historical and Literary Significance

The monastery was dedicated to St. Aonghas (Aengus), the renowned 9th-century scholar associated with multiple Irish ecclesiastical sites including Dysartenos in County Laois. Most significantly, this is the same Aonghas of Tallaght who authored the famous "Féilire Aonghasa" (Martyrology of Aengus), one of the most important works of early Irish religious literature.

According to the late 12th-century preface to the Féilire Aonghasa, Aonghas began his metrical martyrology at nearby Coolbanagher in Laois before finishing it at Tallaght monastery. The 1033 entry in the Annals of the Four Masters records the death of "Conn, son of Maelpadraig, airchinnech [erenagh] of Mungairit [Mungret] and Disert-Oenghusa," linking this monastery administratively with the major foundation at Mungret, 13 kilometers to the north-northeast.

Exceptional Architectural Description

The detailed 1868 architectural survey provides one of the most comprehensive descriptions of any Irish round tower. The structure demonstrates "almost complete symmetry and proportion, the entasis, or batter, being most carefully designed and worked out." Built on a limestone rock foundation that crops slightly above surface level, this firm base accounts for "the perfect solidity of the superstructure, and the absence of all cracks and settlements in the walling."

Construction Materials and Techniques

The tower showcases sophisticated material selection: the main structure uses limestone worked as "roughly squared rubble work with few spawls, stones of large size laid in courses, or nearly so, and accurately dressed to the curve, and batter, of tower." For decorative elements, the builders chose "sandstone, of a bright red colour and fine quality" for door and window dressings, with stones "remarkably well worked and close jointed."

Precise Architectural Measurements

The 1868 survey recorded exceptionally detailed dimensions: the tower rises from a plinth averaging 12 inches (0.3m) high with 4 inches (0.1m) offset, reaching 65 feet 7 inches (20m) from below the doorway to the present eave. The circumference above the plinth measures 54 feet (16.45m), with internal diameter of 7 feet 10 inches (2.4m) at door-sill level where walls are 4 feet 3 inches (1.3m) thick.
The careful tapering shows remarkable precision: the diameter at the top measures 5 feet 6 inches (1.67m) with wall thickness of 3 feet 6 inches (1.1m), giving "the ratio of batter on external face, as about one in twenty-six."

Original Height and Missing Elements

The 1868 observer calculated that the tower originally stood much taller, noting that with walls still 3 feet 6 inches thick at 65 feet height, "it had been originally two storeys higher." Adding 11 feet for each story and 15 feet for the conical roof would have made the complete tower 102 feet (31m) high—ranking among Ireland's tallest round towers.

Interior Organisation

The structure contains five stories divided by four offsets averaging 5 inches (0.12m) wide that supported original floors. Uniquely, "the height of the storeys is regularly graduated from the bottom by about one foot each storey," with the basement story measuring 15 feet (4.57m) high.

Doorway and Windows

The semicircular-headed doorway faces east, positioned 15 feet (4.57m) above ground level. Made from large blocks of red sandstone with converging jambs, it measures 2 feet 9 inches (0.83m) wide at the sill, narrowing to 2 feet 5 inches (0.73m) at the spring of the arch, and standing 4 feet 5 inches (1.35m) high.

Three windows provide interior lighting: an angular-headed window above the second floor (facing an unspecified direction), a semicircular-headed window above the third floor facing south, and a quadrangular window above the fourth floor facing north. Each window shows the characteristic tapering design of Irish round tower openings.

Remarkable Archaeological Excavation of 1849

The excavation by Luke Christy revealed extraordinary evidence of the tower's construction and use history. After removing 3 feet of "decayed vegetables, rubbish, dead birds, &c," excavators found earth with mortar traces and stones down to 5 feet depth. Between 4-5 feet, they discovered human bones "imbedded in the earth, without any trace of coffin or covering whatsoever...in a very decayed state, and belonged to one body."

Complex Floor System

At 6 feet depth, excavators found "a well-made, solid floor of clay, cracked a good deal, with whitish marks through it, over a foot thick, and looking as if it had been subject to fire." Below this was "a sort of red clay filling, and then about two feet from the bottom another floor nearly the same as the first." At the very bottom, "just above the rock we got a quantity of cherry stones."

The foundation was laid directly on rough rock "without any apparent preparation," and additional human remains and a clinker were found below the second clay floor, indicating the site's complex history of use and modification.