The Cáin Adomnáin (Law of Adomnán), also known as the Lex Innocentium (Law of Innocents) was promulgated amongst a gathering of Irish, Dál Riatan and Pictish notables at the Synod of Birr in 697. It is named after its initiator Adomnán of Iona, ninth Abbot of Iona after St. Columba.
According to D.N.Dumville, it is suspected that the promulgation of this law in 697 was a centennial commemoration of Columba, who died in 597. As a successor of Columba of Iona, Adomnán had sufficient prestige to assemble a conference of ninety-one chieftains and clerics from Ireland, Dál Riata, and Pictland at Birr to promulgate the new law. As well as being the site of a significant monastery, associated with Saint Brendan of Birr, Birr was close to the boundary between the Uí Néill-dominated northern half of Ireland, and the southern half, where the kings of Munster ruled. It therefore represented a form of neutral ground where the rival kings and clerics of north and south Ireland could meet.
Various factors, including Marian devotion in seventh- and eighth-century Ireland, are supposed to have contributed to inspire Adomnán to introduce these laws, but it may also be that as Columba's biographer, he was prompted by the Saint's example. It was originally known as the Law of the Innocents, focusing on the beneficiary non-combatants. Upon its renewal in 727, it referenced its author.
The indigenous Brehon Laws were committed to parchment about the 7th century, most likely by clerics. Most scholars now believe that the secular laws were not compiled independently of monasteries. Adomnan would have had access to the best legal minds of his generation.
This set of laws were designed, among other things, to guarantee the safety and immunity of various types of non-combatants in warfare. It required, for example, that "...whoever slays a woman ... his right hand and his left foot shall be cut off before death, and then he shall die."
If a woman committed murder, arson, or theft from a church, she was to be adrift in a boat with one paddle and a container of gruel. This left the judgment up to God, and avoided violating the proscription against killing a woman.
The laws also provided sanctions against the killing of children, clerics, clerical students and peasants on clerical lands; against rape, against impugning the chastity of a noblewoman, prohibited women from having to take part in warfare, and more besides. Many of these things were already crimes, either under the common Irish laws.
The law described both the secular fines which criminals must pay, and the ritual curses to which law-breakers were subject. Bystanders who did nothing to prevent the crime were as liable as the perpetrator. "Stewards of the Law" collected the fine and paid it to the victim or next of kin.
Adomnán's initiative appears to be one of the first systematic attempts to lessen the savagery of warfare among Christians - a remarkable achievement for a churchman on the remote outer edge of Europe. In it he gave local expression, in the context of the Gaelic legal tradition, to a wider Christian movement to restrain violence. 
It was an early example of international law in that it was to be enforced in Ériu and Albu, (Ireland and Britain), although Britain, refers to only what is now northern Scotland, for it was the kings of that region who were guarantors of the Law.
As with later clerical efforts, such as the Peace and Truce of God movement in millennial France, the law may have been of limited effectiveness. Fergus Kelly notes that no cases relating to the Cáin Adomnáin have been preserved. Thus, we do not know whether the harsh penalties which it mandates, which may have contradicted the general character of Irish law, were rigidly enforced.
There are annalistic examples of the justice of the Cáin Adomnáin being applied, such as here by Cinel-Eóghain High King Niall Glúndubh, for whom the O'Neill Clan of Ulster are named.
- In 907 the sanctuary of Ard Macha was violated by Cearnachan mac Duilgen who took a captive from the church and drowned him in Lough Cier nearby.
- This perpetrator was taken by Nial Glundub mac Aedha, Righ an Tuaisceirt, having replaced his brother Domnall as king of the north, and he drowned Cearnachan in the same lake Lough Cier in revenge for the violation of Padraicc.
- Dumville, D.N., "Review" of O'Loughlin's Adomnan at Birr, AD 697: Essays in Commemoration of the Law of the Innocents in the Catholic Historical Review, pp. 283-284, Volume 89, Number 2, April 2003
- "Cain Adomnan", Birr Historical Society
- Cf. Adomnán, Life, II, 24 and II, 25.
- Charles-Edwards, T.M., Early Christian Ireland, p.560, Cambridge University Press, 2000, ISBN 9780521363952
- Grigg, Julianna. "Aspects of the Cain: Adomnan's Lex Innocentium", Journal of the Australian Early Medieval Association, Vol.1, 2005
- "Law of the Innocents", Foghlam Alba
- "Adomnán's Law of the Innocents",Travels in Time
- Kelly, p. 79.
- Kelly, pp.234–235: "the law texts of the Senchas Már collection consistently favour reparation by payment rather than the death penalty for murder and other serious offences (by either sex)."
- Adomnán's Law of the Innocents - Cáin Adomnáin: A seventh-century law for the protection of non-combatants, translated by Gilbert Márkus. Kilmartin, Argyll: Kilmartin House Museum, 2008. ISBN 978-0-9533674-3-6
- Adomnán of Iona, Life of St Columba, edited & translated Richard Sharpe. London: Penguin, 1995. ISBN 0-14-044462-9
- Kelly, Fergus (1988). A Guide to Early Irish Law. Early Irish Law Series 3. Dublin: DIAS. ISBN 0901282952.
- Adomnán at Birr, AD 697: Essays in Commemoration of the Law of the Innocents. Edited by Thomas O'Loughlin. (Dublin: Four Courts Press. 2001)