Flann was the son of Echthigern mac Óengusso, who had been lector at the monastery of Monasterboice (modern County Louth), in Irish Mainistir Buite, whence Flann's byname, meaning "of Monasterboice". He belonged to the Ciannachta Breg, a kindred which, by the turn of the first millennium controlled Monasterboice, providing its abbots and other notables. Flann himself was also fer légind (lit. 'man of textual study', i.e. lector, head of a monastic school) there, as was his father. His son, also called Echthigern (d. 1067), would become superior (airchinnech) of Monasterboice.
Flann's earliest datable works are from the years following the battle of Clontarf (1014), when Máel Sechnaill mac Domnaill of the Uí Néill resumed his reign as High King of Ireland (1014-1022). These are Ríg Themra dia tesbann tnú and Ríg Themrea toebaige íar ttain, which together comprise an Uí Néill-orientated history of the kingship of Tara. Among his other poems, some apparently composed much later in his life, Mide maigen clainne Cuind, deals with Clann Cholmáin, to which Máel Sechnaill belonged, while Mugain ingen Choncraid chain and Síl nÁedo Sláine na sleg both deal with the neighbouring Uí Néill kindred of Síl nÁedo Sláine, sometime overlords over Ciannachta Breg. Flann is also attributed a series of five poems on the kings and martial history of the northern Uí Néill kingdom of Cenél nÉogain and on the legendary origins of Cenél nÉogain's fortress at Ailech.
A number of Flann's poems appear in the Lebor Gabála Érenn—the Book of the Invasion of Ireland—and his works on the Tuatha Dé Danann were influential, while a couple concern world history or themes from classical literature. The most influential was Réidig dam, a Dé, do nim, a lengthy metrical history of the world kings of Eusebian tradition which appears to be related to Bede's Chronica Maiora. During the early modern era, Flann became known as the author of a number of prose synchronistic tracts. While the tracts are authentically medieval, non-circumstantial evidence for Flann's involvement is lacking. Some of these tracts set Irish history within universal history, in the tradition of the Eusebius-Jerome Chronicon. Another presents lists of Irish and Scottish kings arranged to show contemporaries, which is an important source for the early history of Scotland, whether or not Flann is taken as its author.
Despite its mysterious origins, the idea that Flann authored synchronistic material has had a big impact on his subsequent interpretation by scholars. For example, Eoin MacNeill considered Flann to be the first of the synthetic historians; his supposed synthesis of biblical history and foreign world chronicles with Irish annals, myths, and genealogical records was to be much emulated by subsequent writers. More generally, Francis John Byrne has stated that he "...was the leading light among the 'synthetic historians' who shaped what was to remain the official history till the seventeenth century and beyond..." and that "...it was largely on his [Flann's] authority that the official doctrine of the monopoly of the high-kingship by the Uí Néill from the time of St Patrick to the usurpation of Brian became accepted, even by the Munster Annals of Inisfallen." Considering his verse histories of Tara and of various Uí Néill kingdoms, Dauvit Broun has placed Flann within a central medieval historiographical movement towards promoting kingdoms over dynasties as political units; his interest in noting kings' causes of death, for instance, is to do with stressing the "institutional longevity of a kingship in contrast with the mortality of kings."
Flann himself died on 25 November 1056, the date supplied by the Annals of Tigernach. The Annals of Ulster call him "eminent lector and master of the historical lore of Ireland". A quatrain on Flann appears in an anonymous poem on the episcopal court of Áed úa Forréid (bishop of Armagh, 1032–1056), composed between 1032 and 1042; it provides a brief but probably near-contemporary thumbnail sketch of the man.
"Flann, from the famous church of sweet-voiced Buite.
Slow the glance of the eye in his gentle head.
He is a magic mead scholar who imbibes ale.
Final scholar of the three Finns’ land is Flann".
- See also Carey:Flann Mainistrech (d.1056); Ní Mhaonaigh:Flann Mainistrech
- Dobbs, Pedigree and Family
- Carey: Flann Mainistrech (d. 1056)
- Byrne, Ireland and her neighbours, p.866
- Peter J. Smith (ed. and trans.): Mide Maigen
- MacNeill (ed. and trans.): Poems by Flann Mainistrech, Gwynn (ed. and trans.): Ailech II
- Carey: Irish National Origin Legend, pp.18-22; Thanisch: Flann Mainistrech's Götterdämmerung
- For example,Mac Eoin (ed. and trans. [commentary in Modern Irish]): Dán ar Chogadh na Traoi
- Mac Airt (ed. and trans.): Middle Irish poems on world-kingship; Mac Airt (ed. and trans.): Middle Irish poems on world-kingship (suite); and Mac Airt (ed. and trans.): Middle Irish poems on world-kingship (suite). For more information, see here.
- Scowcroft: Leabhar Gabhála Part 1, pp.128 (n.135), 131
- For one such tract, see MacCarthy (ed. and trans.), Codex Palatino Vaticanus No. 830, pp.286–317
- Anderson: List of the Kings, p.109; Broun: Irish Identity, pp.2, 171 (n.23)
- MacNeill: Celtic Ireland, pp.26–41
- Byrne, Ireland and her neighbours, pp.865–866
- Broun, Scottish Independence, pp.39–47
- Murphy (ed. and trans.), A Poem in Praise, p.155
- Anderson, Marjorie O. (October 1949), "The Lists of the Kings", Scottish Historical Review 28:2, vol. 28, pp. 108–118, JSTOR 25525997
- Broun, Dauvit (1999), The Irish Identity of the Kingdom of the Scots, Studies in Celtic History 18, Woodbridge: Boydell, ISBN 978-0851153759
- Broun, Dauvit (2007), Scottish Independence and the Idea of Britain: From the Picts to Alexander III, Edinburgh: Edinburgh University Press, ISBN 978-0748623600
- Byrne, Francis John (2005), "Ireland and her neighbours, c.1014-c.1072", in Ó Cróinín, Dáibhí (ed.), Prehistoric and Early Ireland, A New History of Ireland, vol. I, Oxford: Oxford University Press, pp. 862–898, ISBN 978-0-19-922665-8
- Carey, John (1994), The Irish National Origin-Legend: Synthetic Pseudo History (PDF), Quiggin Lectures on the Sources of Mediaeval Gaelic History 1, Cambridge: The Department of Anglo-Saxon, Norse, and Celtic, University of Cambridge
- Carey, John (2004), "Flann Mainistrech (d. 1056)", Oxford Dictionary of National Biography, Oxford: Oxford University Press, retrieved 2008-03-22
- Dobbs, Margaret E. (1956), "The Pedigree and Family of Flann Manistrech", Journal of the County Louth Archaeological Society, vol. 5:3, County Louth Archaeological and History Society, pp. 149–153, JSTOR 27728169
- Gwynn, Edward J. (1906), "Ailech II", The Metrical Dindshenchas, vol. IV, Dublin: Royal Irish Academy, pp. 101–107
- MacCarthy, Bartholemew (1892), The Codex Palatino-Vaticanus No. 830, Todd Lecture Series 3, Dublin: Royal Irish Academy
- Mac Airt, Seán (1954), "Middle Irish poems on world-kingship", Études Celtiques, vol. 6, pp. 255–280
- Mac Airt, Seán (1956), "Middle Irish poems on world-kingship (suite)", Études Celtiques, vol. 7, pp. 18–45
- Mac Airt, Seán (1959), "Middle Irish poems on world-kingship (suite)", Études Celtiques, vol. 8, pp. 98–119, 284–297
- Mac Eoin, Gearóid (1961), "Dán ar Chogadh na Traoi [Luid Iasón ina luing lóir]", Studia Hibernica 1, Studia Hibernica, Editorial Board, St. Patrick's College, Drumcondra, a College of Dublin City University, pp. 19–55, JSTOR 20495675
- MacNeill, Eoin (1913), "Poems by Flann Mainistrech on the Dynasties of Ailech, Mide and Brega", Archivium Hibernicum 2, vol. 2, Catholic Historical Society of Ireland, pp. 37–99, JSTOR 25529575
- MacNeill, Eoin (1921), Celtic Ireland, Dublin: Lester
- Murphy, Gerard (1944), "A Poem in Praise of Áodh Úa Foirréidh, Bishop of Armagh (1032‒1056)", in O'Brien, Sylvester (ed.), Measgra i gCuimhne Mhichil Uí Chléirigh, Dublin: Assisi Press, pp. 140–164
- Ní Mhaonaigh, Máire (2005), "Flann Mainistrech", in Duffy, Seán (ed.), Medieval Ireland: An Encyclopaedia, London: Routledge, pp. 303–306, ISBN 978-0415940528
- Scowcroft, R. Mark (1987), "Leabhar Gabhála. Part I: The Growth of the Text", Ériu 38, vol. 38, Royal Irish Academy, pp. 81–142, JSTOR 30007523
- Smith, Peter J. (2004), "Mide Maigen Clainne Cuind: A Medieval Poem on the Kings of Mide", Peritia 15, doi:10.1484/J.Peri.3.433
- Thanisch, Eystein (2012), "Flann Mainistrech's Götterdämmerung as a Junction within Lebor Gabála Érenn", Quaestio Insularis 13
- Thanisch, Eystein (2015), The Reception and Use of Flann Mainistrech and His Work in Medieval Gaelic Manuscript Culture (PhD), University of Edinburgh, hdl:1842/15763