Kingdom of Midhe
Perhaps the most radically different kingdom at the start from the familiar medieval setup is the realm of the Southern Ui Neill, the Kingdom of Midhe. In 479, the conquests of the Ui Neill begun several decades earlier had yet to be completed, and the Ui Neill under King Lugaid mac Loegaire rule over only about half of what would eventually form the full extent of the kingdom.
At the eastern edge of the kingdom, at the frontier against the fading old tribes of the area are the sons of High King Loegaire, the Cenel Loegairi, including the current king Lugaid and another branch ruled by his brother. This would be the high point of their success; after the death of Lugaid they would decline to become vassals of the Sil nAedo Slaine, according to a curse from St. Patrick himself in the legends. Perhaps you can defy this curse and ensure there are many more Kings and High Kings of your house.
Ruling from the ancient and sacred Hill of Uisneach is Conall Cremthaine mac Niall, whose descendants would later come to dominate all of Midhe. Under him is the relatively minor kingdom of the Cenel Maini, ruled by Maine mac Niall, with a suspiciously similar name and nearby location to the larger and more important Ui Maine in Connacht. Seize Brega and the kingship and cement your control over the other dynasties of the Southern Ui Neill as Conall Cremthaine, or perhaps make a name for yourself and try to reunite with your brothers across the Shannon River as Maine.
To the north is the is the great kingdom of Breifne, ruled not by the Ui Briuin who would conquer the area in the 8th century but by the Cenel Cairpre, the descendants of the current king Coirpre mac Niall who may have even been High King at some point. Uniting the two main kingdoms of the Ui Neill into a single contiguous arc, the kingdom would slowly decline and lose territory to various Connacht tribes over the next couple of centuries, leaving only two minor kingdoms at each edge to mark its former extent. Hold your kingdom firm and resist the trepidations of invading tribes from the west and other Ui Neill branches envious of your power, and maybe the Ui Neill both North and South can be united under your rule. Under the Cenel Cairpre we also have two minor and rather obscure tribes, the Dartraighe and Masraige.
Only one of the lines of the sons of Niall remains unlanded at start- the Cenel Fiachach, the descendants of Fiachu mac Neill, as their kingdom would not be founded until the defeat of the Ui Failghe in 514. To make this somewhat vaguely probably to happen, Fiachu will start with a strong claim to the province, and the other lords of Midhe also have claims to encourage historical expansion of the Ui Neill branches into the unconquered lands of Midhe.
Finally, in the east of Brega lies the independent Kingdom of the Ciannachta, an unrelated dynasty with distant ties to the Eoganachta. The Ciannachta appear to have been the original rulers of Brega with a substantial kingdom prior to the rise of the Ui Neill, but have been slowly pushed back, eventually to be taken over entirely in the 6th century. Under the Ciannachta is the closely related tribe of the Gailenga, and once there was also the other affiliated tribe of the Luighne, though by this point the Luighne of Brega had been conquered by the Ui Neill. Can you stand your ground and resist the powerful rising tide of the Ui Neill, or will you fall to the inevitable and be doomed to irrelevance as a minor vassal tribe?