Those searching for added understanding of Mannion and O'Mannion ancestry are encouraged DNA test and to join the Mannion project. Variant spellings are also likely to be genetically related. At this time, those who have tested Y-DNA, mtDNA,X-DNA, and autosomal DNA should feel welcome. The best results for the surname study are expected to come from male Y-DNA Mannion participants.
The Mannion and O'Mannion surname has been subject to different spellings in records. A surname project was commenced in July 2016 to permit others whose roots are Mannion or O'Mannion along with different spellings in the records to collaborate, discover relationships, and confirm Mannion roots. Many Mannion alternate spellings have been retained especially among the diaspora. In some cases, Mannions are recorded as Manning. In particular this has been the case in County Galway as well as in the U.S.
For an interesting interview about the Clan with Dr Joe Mannion you might listen to this link.
The Mannion family name is an anglicised form of Ua/Ó Mainnín, signifying ‘descendant of Mainnín’, an early tenth-century king of the Sogain people of East Galway. This ancient clan can trace their roots in the medieval Irish genealogies to the Cruthin, the earliest band of 'Celtic' settlers deemed to have reached Ireland about the sixth century BC. An Ulster prince called Sogan Sálbuide is reported to have been granted an extensive territory in Connacht between the Clare river and the Suck by the legendary Queen Medb about the time of the birth of Christ. Following the relocation of the Uí Maine people to the area about AD 500, the kingdom of Sogan became a sub-kingdom of Uí Maine (later Hy Many), and its Sogain inhabitants were obliged to render tribute and military service to the Uí Maine overkings.
As part of their martial obligations, the Sogain (under the customary leadership of their Ó Mainnín king) formed part of the household troops of Tadg Ó Cellaig at the battle of Clontarf in 1014, and played a pivotal part in the historic defeat of the Norsemen and their Leinster allies by the celebrated high-king Brian Boru. Likewise, under the year 1135 the annals inform us that ‘Ó Mainnín, king of Sogan’ was slain at the battle of Áenach Máenmaige (now Finnure in the parish of Abbeygormacan) while fighting with the Uí Chellaig against the Uí Chonchubair of Connacht. Again, the incumbent Sogain ruler Murchad Ó Mainnín was slain in a similar confrontation at Ruba Gealain in Co. Roscommon in 1180, while a later Ó Mainnín chieftain was killed in like circumstances at the battle of Roscommon in 1377.
The Ó Mainnín clan and their ancestral Cenél nDéigill sept were originally located to the west and south of the Killaclogher/Abbert river, in an area comprising the parishes of Athenry and Monivea, together with parts of Knockmoy, Kilmoylan and Lackagh. Encroachment eastwards over the Clare river by the Uí Briúin of Mag Seóla led to the loss of lands later included in Kilmoylan and Lackagh, while Athenry had been lost to the Clann Taidg sept of Síl Muiredaig extraction by the mid-twelfth century. Finally, about the middle of the fourteenth century, Diarmaid Ó Cellaig captured the Ó Mainnín stronghold at Clogher (now Killaclogher), dispossessed the clan of their lands in the parish of Monivea, hanged the Ó Mainnín chieftain, and forced his kinsmen to migrate eastwards over the Killaclogher river. Here they dislocated some of the Uí Chon Chenainn of Uí Diarmada and later held most of the parish of Killoscobe, the central portion of Moylough, and two townlands in Knockmoy.
After their relocation, the ruling branch of the Ó Mainnín clan established their power centre at Menlough, originally Mionlach Uí Mhainnín, 'the small lough of the Mannions', where a substantial castle with bawn and associated medieval village was erected in the fifteenth or sixteenth century. A collateral branch of the family seemingly erected Clooncurreen Castle about the same time. Ó Mainnín landholders continued to own and occupy much of their newly-acquired patrimony until the mid-seventeenth-century Cromwellian confiscations, at which time they were dispossessed and dispersed, with just small portions of the clan lands being restored to its former owners during the Restoration under Charles II (1660-1685). To this day, the Mannion clan continues to be strongly represented in the East Galway area.
Source: Dr Joe Mannion, http://www.mannionclan.org/