[[Íomhá:Www.wesleyjohnston.com-users-ireland-maps-historical-map1014.gif|mion|lár|250px|An tOirialla roimh [[Ionradh na Normannach]]]]
De réir an tseanchais, bunaíodh an tOirialla ag [[Colla Uais|na Trí Cholla]],<ref name="ConnollyPg12"/> ''a whodeirtear aregur saidchloígh tosiad havelár conqueredna what is now centraln [[Ulaidh Uladh]] from theóna ''h[[Ulaid]]'' féin. ''The decisive victory was the battle of ''[[Achaidh Leithdeircc|Achadh Leithdheirg]], saida tomheastar havegur beentharla foughtsé aroundtimpeall thena yearbliana 331. However,Creidtear thisanois taleámh isgur thoughtficsean toé bean mostlyscéal fiction,seo andden thechuid actualis yearmó, andcircumstancesNí offios howdá thelaghad Airgíallaconas confederationnó camecathain abouta isbunaíodh cónaidhm an unknownOirialla.
Faoi cheannas na ''nUlad'' san oirthear a cheaptar a bhíodh siad o thús, ach ón 6ú haois ar aghaidh, bhain de réir a chéile [[Uí Néill an Tuaiscirt]] ó thuaidh dá gcuid críoch,<ref name="DuffyEncyclopediaPg14"/> chomh maith le h[[Uí Néill an Deiscirt]] ó dheas. Ón bhliain 735, thit siad faoi cheannas an [[Cenél nEógain|Chineáil Eoghain]], agus faoin mbliain 827, bhí siad ina vasáilligh.<ref name="DuffyEncyclopediaPg14"/>
Originally thought to have been under the dominance of the neighbouring Ulaid to the east, the territory of the Airgíalla from the 6th century onwards was gradually eroded by the encroachment of their northern neighbours, the Cenél nEógain of the [[Uí Néill an Tuaiscirt]],<ref name="DuffyEncyclopediaPg14"/> as well as the [[Uí Néill an Deiscirt]] to their south. From 735 they fell under the dominance of the Cenél nEógain, and by 827 had become their vassals.<ref name="DuffyEncyclopediaPg14"/> The Ríocht an Oiriall was at its peak in the 12th century, under king [[Donnchad Ua Cerbaill]]. The later constricted kingdom of Oiriall survived in Monaghan under the [[McMahon|Mac Mathghamhna]], until the end of the [[Gaelic Ireland|Gaelic order in Ireland]].<ref name="ConnollyPg12"/>
Bhain Ríocht an Oiriall barr a réime amach sa 12ú haois faoi rí ''[[Donnchad Ua Cerbaill]]''. Mhair ríocht laghdaithe fós i Muineachán faoi [[Mhic Mhathghamhna]] go dtí deireadh ré na [[Gaeil|nGael]] i nÉirinn.<ref name="ConnollyPg12"/>
