The town and the castle take their name from the geography. Cromadh in Irish means 'bend' or 'curve' - the River Maigue here makes a tight, ornate loop, and a fortress built on the high inside of that loop has river protection on three sides without needing to dig a moat. Before the Normans came, this country belonged to the Uí Cairpre Áebda, a branch of the larger Uí Fidgenti kingdom, and their leading family were the O'Donovans. According to the seventeenth-century antiquarian Samuel Lewis, one Dermot O'Donovan built the first known fortress at Croom during or shortly before the reign of King John of England - securing land the O'Donovans had taken from their MacEniry cousins. The name appears even earlier: a 1130s manuscript called the Caithréim Chellacháin Chaisil mentions an O'Donovan ancestor associated with Cromadgh as far back as around the year 960.

The political map of Munster turned in 1194 with the death of Domnall Mór Ua Briain, the last great king of Thomond. The Normans took advantage. By around 1200 the FitzGeralds were granted huge swathes of what had been Uí Fidgenti land, and Croom became one of their principal residences. The current tower castle - the one whose ruins still stand - was probably built in the early thirteenth century, after attacks like the one recorded in 1151, when Ruaidhri, son of Toirdhealbhach Ua Conchobhair, raided into Thomond and burned Cromadh, carrying away many cows. The new tower was meant to make such raids harder. It stood for centuries, the FitzGerald banner of the Earls of Kildare flying from its battlements, with the family's chosen battle cry rising from its garrisons. One branch of the O'Donovans, displaced as lords but never quite gone, kept farming the surrounding land - they still hold a farm at Croom that has been in their family for more than four hundred years, though they have not lived in the tower since the FitzGeralds moved in.

How does a place name become a battle cry? Through repetition, in moments of danger. When a FitzGerald lord rode out from Croom Castle to fight his rivals, his men shouted the name of their stronghold - 'Crom! Crom!' - and the suffix abú meaning 'forever' or 'to victory' attached itself. The cry travelled with the family. The Earls of Kildare carried it into the Wars of the Roses-era Irish politics, into the rebellion of Silken Thomas in 1534, into the centuries of FitzGerald power and FitzGerald downfall. By the eighteenth century the cry had become a heraldic motto, embroidered on the FitzGerald arms. The Dukes of Leinster, descended from the same FitzGeralds, still use it. Most of the people who recite the motto today have never heard of the small Limerick town that gave it meaning.

Today Croom Castle is, as the antiquarian record honestly puts it, a large undistinguished ruin. Two walls of the tower stand to half their original height, weather-beaten and shaggy with ivy. The other two have collapsed into a heap of mortar and stone that local farmers have, over the centuries, partly carted off for field walls. There is no formal visitor centre. The castle sits on private land near the modern town of Croom, where the N20 traffic between Limerick and Cork rumbles past a few hundred metres away. What is left to see is not the building but the site - the bend in the river, the strategic high ground, the way the water curves protectively around the ridge. Eight centuries ago this was the seat of one of the most powerful families in Ireland. The river still curves. The cattle still graze. The cry is still on the Leinster arms.