It was founded in the 13th century during the Anglo-Norman colonization. Much of the medieval town wall (1211) survives, together with the keep of the castle (1235) and part of the Dominican priory (founded 1241), which was specifically exempted from Henry VIII's dissolution of the monasteries.
There was an Irish settlement at Athenry on the Clareen river probably as far back as the seventh or eighth century. In about 1235, Meiler de Bermingham founded a castle there, surrounded by a roughly D-plan stone enclosure with round towers on the corners. Inside the enclosure he raised a low-level, half-type great tower, 16.4 x 10.3 metres (54 x 34 ft), of one story at first-floor level set over a pronounced splayed plinth which surrounded a basement. Within a generation, Meiler's son Piers raised the height of the first floor, lifting its ceiling and walls, and embellishing its entrance with the fine arched door at the south-east end. This was reached from outside by a staircase from the ground, probably of timber (a reproduction stair exists there today), and it was protected by some sort of fore-building. At the same time, he raised a banqueting hall along the east wall of the enclosure, which used the enclosure wall for its fourth side. Among the decorations Piers inserted were narrow windows with trefoil heads, which are some of the very few such castle windows anywhere in Ireland (Ferns, Wexford, and Lea, Laois, are two other examples).
In the fifteenth century, the tower was raised yet again to provide two more floors including an attic between two new gable ends. The basement, meanwhile, which had hitherto been accessible only by ladder through a trap in the first floor, now received its own entrance cut into the splayed plinth. The top of the tower was equipped with battlements whose rectangular merlons were more akin to Norman-style merlons such as were found in English great towers.
This interesting castle has been very carefully restored by the Office of. Public Works. It was linked to the town that expanded beside it during the Middle Ages, which itself was enclosed by a comprehensive system of walls, towers and gates, much of which remains to be seen today. Among the buildings enclosed was a Dominican friary (now in ruins) which was founded soon after Meiler's first Works