His brothers were Tighearnán, Brian (d. 1337), Aedh (d. 1351), Seán, Conchobhar, Fearghal Mág Samhradháin who also became chief and died in 1393, Maghnus Ruadh Mág Samhradháin who also became chief and died in 1408, Giolla na Naomh, Tadhg Buidhe (d. 1367), Maghnus Buidhe (d. 1357) and Matha (d. 1356).
The Annals of Ulster for 1354 state- Maghnus Mag Samradhain the Tawny was killed in the Route of Mac Uidhilin by Aedh Ua Neill.
The Annals of the Four Masters for 1357 state- Brian, son of Gilchreest O'Rourke, and Manus Boy Magauran, were slain in the Route, Mac Quillin's territory, by Hugh O'Neill.
The Annals of Loch Cé for 1357 state- Brian, son of Gilla-Christ O'Ruairc, and Maghnus Buidhe Mac Samhradhain, were killed in Ruta-Mic-Ugilin, by Aedh O'Neill, in hoc anno.
The Annals of Connacht for 1357 state- Brian son of Gilla Crist O Ruairc and Magnus Buide Mag Samradain were killed by Aed O Neill in the Route this year.
John O'Doherty, Chief of Ardmire, Owen Connaghtagh, and Turlough Mac Sweeny, were taken prisoners on this occasion by the son of O'Conor, and many persons were slain by him.
The Annals of Connacht for 1359 state- Cathal Oc O Conchobair inflicted a severe defeat, on the Cenel Conaill at Ballyshannon, where Seoan O Dochurtaig, chieftain of Ardmire, and Eogan Connachtach and Toirrdelbach Mac Suibne were captured and great slaughter was made.
The Annals of Clonmacnoise for 1359 state- Cahall oge O'Connor gaue an overthrow to the Inhabitants of Tyreconell at Belaseannye, where John O'Dochorty, chieftaine of Ardmire and Terlaugh m'Swynie were taken, and a great many of others slaine besides.
According to the Book of Magauran, Niall was handsome, with curly hair, dark eyebrows, blue-grey eyes, red lips and white teeth.
His wife Sadhbh had blue eyes, black eyebrows and curly long blond hair which was braided and reached to her feet.
She did embroidery on satin and depicted lions, dragons, horses, birds and herds of oxen fleeing from a golden wolf.
Niall married Sadhbh (d. 1373), the daughter of Cathal mac Domhnall Ó Conchobair (d. 1324), King of Connacht from 1318 to 1324, and his wife Ailbhe.
Flaithbheartach O'Rourke's sons, some or all by Sadhbh and therefore Niall's step-sons, were Maelsechlainn (d. 1401), Domhnall, CúChonnacht, Fergal, Tighernán na Lurgan, Art and Concobhar.