The First Battle of Athenry was fought on the 15th August 1249. Ath an Ri had existed as a minor settlement prior to its foundation as a town by the de Bermingham family in 1241, and was thus still little more than a military base in (highly) hostile territory.

The victor of the day was Jordan de Exeter , Sheriff of Connacht. The Four Masters give the following account of it:

"An army was led by the Roydamnas heirs presumptive of Connaught, namely, Turlough and Hugh, two sons of Hugh, the son of Cathal Crovderg, to Athenry, on Lady Day in mid-autumn, to burn and plunder it. The sheriff of Connaught was in the town before them, with a great number of the English. The English demanded a truce for that day from the sons of the King of Connaught, in honour of the Blessed virgin Mary, it being her festival day; but this they did not obtain from them; and although Turlough forbade his troops to assault the town, the chiefs of the army would not consent, but determined to make the attack, in spite of him."

"When Jordan and the English saw this, they marched out of the town, armed and clad in mail, against the Irish army. The youths of the latter army, on seeing them drawn up in battle array, were seized with fear and dismay, so that they were routed; and this was through the miracles of the Blessed Virgin Mary, on whose festival they had refused to grant the truce demanded from them. Of their chiefs were here killed Hugh, son of Hugh O'Conor; Dermot Roe, son of Cormac O'Melaghlin, the two sons of O'Kelly; Brian an Doire, the son of Manus; Carragh Inshiubhail, son of Niall O'Conor; Boethius Mac Egan; the two sons of Loughlin O'Conor; Donnell, son of Cormac Mac Dermot; Finnanach Mac Branan; Cumumhan Mac Cassarly, and others besides."

Source

http://www.ucc.ie/celt/published/T100005C/index.html