In histories written after Brian Bóruma of the Dál gCais of Munster had become High King of Ireland, Cathal too was presented as High King, one of only three historical Munster kings—the third was Feidlimid mac Crimthainn—for whom this claim is made in southern sources. The historical record discredits these claims.
Cathal's conflict with the Uí Néill kings, Fergal mac Máele Dúin, Flaithbertach mac Loingsig, and Áed Allán, son of Fergal mac Máele Dúin, is reported at some length in the Irish annals, and again northern and southern versions provide differing accounts. Cathal also appears as a character, not always portrayed sympathetically, in a number of prose and verse tales in the Middle Irish language.
The Eóganachta kingship, which had its chief seat at Cashel and chief church at Emly, was the most powerful in the southern half of Ireland—Leth Moga—while the various branches of the Uí Néill dominated the northern half—Leth Cuinn. The chief place of the Uí Néill kingship was at the Hill of Tara which gave the High Kingship of Ireland its alternative name, King of Tara. The succession generally alternated between the northern and southern branches of the Uí Néill.
For the century and a quarter until Cathal's death, the kingship at Cashel was dominated by the Glendamnach and Cashel septs. The lands of the Glendamnach lay to the south-west of Cashel, in the middle valley of the Blackwater. Cathal's father, Finguine mac Cathail Con-cen-máthair(d.696), uncle, Ailill mac Cathail (d. 701), grandfather, Cathal Cú-cen-máthair (d. 665/666), and great-grandfather, Cathal mac Áedo (d. 628), had been kings of Munster.
Cathal's immediate predecessor was probably Cormac mac Ailello of the Cashel sept, who was killed in battle against the Deisi in 713. Eterscél mac Máele Umai, who had been king and did not die until 721, had probably abdicated much earlier, so that Cathal was king at Cashel from around 713 onwards.
While the Uí Néill and Eóganachta were the most important kingships in Ireland, the kings of Leinster and the kings of Connacht were significant forces. Leinster, once a much larger region, the northern parts of which had been conquered by the Uí Néill, was the target of expansionist Uí Néill kings, and also of the Eóganachta. The contest for control of Leinster would play a major part in Cathal's reign, and indeed in relations between the Eóganachta and Uí Néill in the centuries which followed. The kings of Connacht claimed a common kinship with the Uí Néill, and were largely favourable towards them. The remaining provincial kingship, that of the kings of Ulster (Ulaid), controlled a much smaller area than the later province of Ulster, largely confined to the lands north and east of Lough Neagh, and was generally hostile to the Uí Néill.
The Annals of Inisfallen, as partisan a southern record as the Annals of Ulster are biased towards the Uí Néill, give a different and less reliable report of the events in 721:
The harrying of Brega by Cathal son of Finnguine, king of Mumu, and after that he and Ferga son of Mael Dúin, king of Temuir [Tara], made peace; and Ferga submitted to Cathal. For these were the five kings of the Munstermen who ruled Ireland after the [introduction of the] Faith, viz. Aengus son of Nad Fraích, and his son, i.e. Eochaid who ruled Ireland for seventeen years, and Cathal, son of Finnguine, and Feidlimid, son of Crimthann, and Brian, son of Cennétig.
Fergal led an Uí Néill army south into Leinster again in 722, but this time he was defeated and killed by the Leinstermen. This defeat was record in the Cath Almaine, a poem about the battle of Allen, fought on 11 December 722, the feast of Saint Finnian of Clonard. Much of the work is devoted to the story of the faithful bard Donn Bó, but the introduction provides a late view of the war:
For a long time there was great warfare between Cathal son of Findguine, king of Leth Mogha, and Fergal son of Máel Duin, king of Leth Cuinn. Fergal son of Mael Duin raided Leinster in order to injure Cathal son of Findguine; so Cathal son of Findguine wasted the whole of Magh Bregh [the plain of Brega], until they made peace and truce.
This truce, the poet tells, was broken by the Leinstermen:
The Leinstermen had delivered this battle of Allen in the absence of Cathal mac Finguini, and Cathal was grieved that the battle was fought while he himself was away. They heard of Cathal's grudge against them, so this was the counsel they framed, to carry to Cathal Fergal's head as a trophy of the action.
With the Uí Néill kings no great threat during the reigns of Fogartach, Cináed and Flaithbertach, Cathal sought to extend his authority over Leinster. The Cath Almaine claims that the dispute arose because Fergal mac Máele Dúin had been killed in defiance of the truce he had made with Cathal.
A battle between Mumu and Laigin, in which many of the Laigin and well nigh countless Munstermen perished; Cellach son of Faelchar, king of Osraige, fell therein, but Cathal son of Finnguine, king of Mumu, escaped.
In 733 Cathal raided the lands of the Southern Uí Néill, but was defeated and driven off from Tailtiu by Domnall Midi of Clann Cholmáin. Cathal had more success against the neighbouring Clann Cholmáin Bicc, ruled by Fallomon mac Con Congalt, whom he defeated at the Hill of Ward. In 734 Cathal inflicted a defeat on the Leinstermen at Bealach Ele.
Best of the women of Inis Fail
is Mór daughter of Áed Bennan.
Better is Fingen than any hero
that drives about Femen.
When Fingen died, the story says, Mór Muman married Cathal mac Finguine. Unfortunately, the collector of this tale mistook this Cathal for his grandfather, Cathal mac Áedo Flaind. He may have married Mór Muman, but Cathal mac Finguine certainly did not.