Grennan castle takes its name from the original town name, later changed to Thomastown.
It was erected in 1210 by Thomas FitzAnthony on the site of a previous motte-and-bailey. As a matter of fact a ditch can still be seen around the mound where the keep stands on.
The building measures 18 metres of length and 13 metres of width with windows on the upper floor on all sides and openings at the ground level. The main doorway was on the east side, on the first floor, so the keep was quite impregnable. The walls are about three metres thick. At the ground floor there are three chambers side by side with vaulted ceilings. The great hall was on the first floor.
The castle was taken in 1650 by Oliver Cromwell's army, the garrison left the castle after a two-day siege.
Apparently it was in good conditions until the begining of the 19th century, when it was abandoned. The current owner uses the keep as a shed for cattle and in order to allow them an easy access and shelter some parts of the walls have been knocked down.
Grennan castle is past the GAA grounds to the south of Thomastown. There's a fence around the field, with a stile.
From here it's a 180 metres long walk to the southeast.
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