The FitzGeralds, one of the most powerful Anglo-Norman dynasties in medieval Ireland, rallied their forces with the shout "Crom-a-Boo", a battle-cry drawn directly from Croom itself. That a modest Irish town should echo through centuries of dynastic conflict is curious enough, but the town's own layout holds an unresolved puzzle: nobody is quite certain where the medieval borough actually sat.

The settlement predates the Normans. The Annals of the Four Masters record that a place called Cromadh was burned in 1144 by Toirdealbach Ua Conchobair, the powerful king of Connacht, though the nature of that early settlement remains unknown. In 1215 the manor was granted to Maurice FitzGerald, and Croom became one of the principal FitzGerald manors, remaining in the hands of the FitzGerald earls of Kildare for several centuries. It was confiscated following the rebellion of Silken Thomas, the informal name given to Thomas FitzGerald, tenth Earl of Kildare, who rose against the crown in 1534, and by 1547 the town had passed to the earl of Desmond. The borough's origins are equally obscure. A murage grant, which was a royal licence allowing a town to collect tolls for the purpose of building or maintaining its walls, was issued in 1310, suggesting Croom was no minor place at that point. Burgesses are mentioned in the justiciary rolls as early as 1295, and a rent entry from 1331 records payment of 11 shillings and 9½ pence "for the two parts of their borough", though whether that sum represents the full rent or only a portion is unclear.

The deepest uncertainty concerns the town's physical geography. The castle and church sit on the west bank of the Maigue, yet the present-day street plan of the village, which occupies the east bank, appears to be of relatively recent origin. Scholars have been unable to determine with confidence on which side of the river the medieval borough was centred. Visitors walking through Croom today are, in effect, moving through a palimpsest whose older layers have not yet been fully read. The river itself is worth following, and the contrast between the two banks is quietly telling, even if the documentary record refuses to settle the question.