Irish History From The Annals
In Medieval Ireland it was customary for an under king to surrender hostages to an over king.
The hostages symbolised the subordinate position of the under king and acted as a guarantee of good behaviour. Often the hostages were members of the family of the under King. For example the Annals of Four Masters record that in 1169 Diarmaid Mac Murchadha gave his son as hostage to the High King, Ruaidhri Ua Conchobhair.
The King of Ireland, Ruaidhri Ua Conchobhair, afterwards proceeded into Leinster; and Tighearnan Ua Ruairc, lord of Breifne, and Diarmaid Ua Maeleachlain, King of Teamhair, and the foreigners of Ath-cliath, went to meet the men of Munster, Leinster, and Osraigh; and they set nothing by the Flemings; and Diarmaid Mac Murchadha (Dermot MacMurrough) gave his son, as a hostage, to Ruaidhri Ua Conchobhair.
The lives of hostages were precarious, if the under king acted against the interest of his overlord an immediate reprisal could be taken by injuring or killing the hostages. For example in 1170, after the fall of Dublin, Ruaidhri Ua Conchobhair killed Diarmaid Mac Murchadha’s son and grandson who he held as hostages.
The hostages of Diarmaid Mac Murchadha were put to death by Ruaidhri Ua Conchobhair, King of Ireland, at Ath-Luain, namely, Conchobhar, son of Diarmaid, heir apparent of Leinster, and Diarmaid's grandson, i.e. the son of Domhnall Caemhanach, and the son of his foster-brother, i.e. O'Caellaighe.
On this occasion the killing of the hostages would rebound to the disadvantage of the over King. For by killing the last remaining direct heirs of Diarmaid Mac Murchadha to the Kingship of Leinster, Ua Conchobhair had cleared the way for Mac Murchadh’s Anglo-Norman Son-in-Law, Strongbow, to assume the Lordship of Leinster on Mac Murchadh’s death in 1171.