Guaire Aidni (‘of Aidne’) (d. 663), son of Colmán and overking of Connacht, was an early representative of the dynasty of Uí Fhiachrach Aidni. His father Colmán, who features in the regnal lists as overking of Connacht, was slain in 622 by an Uí Briúin rival, Rogallach son of Uatu, who subsequently laid claim to provincial supremacy. Guaire's mother, according to the ‘Banshenchas’ (lore of women), was one Adamair Deilgneach of the Tradraige, a population group that resided along the River Fergus in what is now Co. Clare. This tradition seems more plausible than that related in the genealogies of the saints, which names his mother as Rím daughter of Fiachra of Éoganacht Locha Léin, and makes him a half-brother of Caimín (qv), the saint of Inis Celtra, of another holy man named Comgán, and of Crimthann Cualann (qv), overking of Leinster. Guaire did have at least one brother, Laidcnén (the Book of Leinster calls him ‘Loingsech’).
The tradition of a marriage alliance with the Éoganachta arises again in relation to Guaire himself; the ‘Banshenchas’ claims that he married Deog, daughter of Fíngin (d. 619) king of Cashel, and (subsequently?) Órnait daughter of Cúán of Éoganacht Áine, who had previously been the wife of Fíngin's son Sechnassach. Five sons of Guaire feature in the record: Muirchertach Nár, Artgal, Cellach, Airmedach, and Congal. He also had at least two daughters: Créide, who became the wife of his ally, Marcán son of Tomán, king of Uí Maine; and Geilgés, who was married off to Cú Chongalt son of Éogan of the northern Uí Fhiachrach.
When their father was slain in 622, Guaire and Laidcnén, still mere youths, were apparently unable to prevent Rogallach from attaining supremacy in Connacht. The brothers seem to have maintained their dynasty's sway over the realms of Corco Mruad and Corco Baiscinn (in western Co. Clare), and Guaire pursued ambitions deeper into Munster. He was defeated in 627 at Carn Feradaig (Cahernarry, Co. Limerick) by the Éoganacht king Fáilbe Flann (qv). Perhaps it was at this stage, as part of a peace arrangement, that Guaire married Deog, who was Fáilbe's niece.
It was not till 649, when Rogallach was slain, that Guaire and Laidcnén made a bid for power at provincial level. That year, at Carn Conaill (near Gort, Co. Galway), Guaire failed to halt an incursion by the king of Tara, Diarmait Ruanaid (qv) son of Áed Sláine (qv), with both sides suffering heavy losses. This setback, however, does not seem to have prevented the brothers from attaining the overkingship of Connacht. The regnal list credits Laidcnén with a reign of only three years, and Guaire with twelve, which suggests that the latter succeeded in 652, but in his obit at 656 (Ann. Tig.), Laidcnén is styled ‘king of Connacht’. Guaire's ally and son-in-law, Marcán son of Tomán, was slain by Uí Briúin in 653, but the following year Uí Fhiachrach managed to defeat this rival dynasty with the loss of its king, Muirgius son of Rogallach. This victory quite possibly paved the way for Guaire to succeed as provincial ruler, and may explain the ‘Banshenchas’ tradition according to which Guaire's widowed daughter Créide became the wife of Muirchertach son of Muirgius.
Guaire features prominently in hagiography and in the king-tales, reflecting the extent to which his reign became encrusted with legend. A saga concerning the battle of Carn Conaill takes up the theme of Guaire and Caimín of Inis Celtra being half-brothers, and presents the saintly Cummian Foto (qv) as acting as envoy for the overking of Connacht. Another story portrays Guaire as helping the holy man Colmán son of Duí to found Cell maic Duach (Kilmacduagh), in the south of Uí Fhiachrach. The Life of Mo-Chuille of Tulla (Co. Clare) claims that Guaire, after his defeat by Diarmait Ruanaid, sought to do penance by donating land to the saint. Even if the association of Guaire with these seventh-century saints is not strictly historical, there is some basis for the episodes concerned; at least two eighth-century abbots of Tuaim Gréine (Tomgraney, Co. Clare, opposite Inis Celtra) were descendants of one Guaire – possibly Guaire Aidni – and there were later Uí Fhiachrach claims in relation to Tulla.
Certain stories of medieval or early modern date feature Guaire in episodes that are quite unhistorical; he is uncharacteristically a villain in the late and fabulous Life of Cellach (qv) of Killala, where he is the instigator of the saint's murder. Another improbable anecdote portrays Guaire, who according to tradition was a paragon of generosity, as the victim of unreasonable demands from the poet Senchán Torpéist (qv); the king is saved from the threat of satire by a saintly hermit named Marbán who, for the purposes of this particular tale, is supposedly another of his brothers. Guaire's daughter Créide also features in a story, and is cast as the lover of Cano (qv) son of Gartnait.
Guaire Aidni died in 663, and according to tradition was buried in Clonmacnoise. His dynasty did not manage to hold off the rising tide of Uí Briúin for long. His sons Cellach and Muirchertach Nár (the latter styled ‘king of Connacht’ in Chronicon Scottorum) died in quick succession, in 666 and 668 respectively. Airmedach died in 675 and Congal ten years later. Of Guaire's grandsons, only Fergal Aidni, son of Artgal, attained overkingship of Connacht. The later kings of Uí Fhiachrach came from Fergal Aidni's line, or from that of his brother Áed son of Artgal.