Short answers, pulled from the story.
The scribe Ruaidhrí Ó Luinín wrote the Annals of Ulster in the late 15th century. He worked under the patronage of Cathal Óg Mac Maghnusa on Senadh-Mic-Maghnusa island, now known as Belle Isle near Lisbellaw on Lough Erne within the kingdom of Fir Manach.
The manuscript contains entries spanning from 431 AD to 1540 AD. Entries up to 1489 AD were compiled by Ruaidhrí himself during this specific period while later additions extending to 1540 AD were written by other hands after his initial work concluded.
Máel Sechnaill mac Máele Ruanaid appears repeatedly between 847 and 879 AD and ruled the southern Ui Neill clan from 846 until his death in 862. Another king named Aed mac Neill also appears in numerous entries including 855.3 and 879.1 with a poetic lament recorded about his passing.
The first mention of Vikings appears in entry 794.7 describing devastation across British islands. Norsemen established a permanent base in Áth Cliath or Dublin by 841 and used naval camps at Linn Duachaill and Duiblinn to plunder Tethba peoples and take hostages from Laigin and Uí Néill states.
Trinity College Library in Dublin holds the original manuscript of the Annals of Ulster while the Bodleian Library at Oxford possesses a contemporary copy filling gaps missing from the primary source. This Oxford copy dates to the early 16th century according to catalog records and is designated as MS. Rawl. B489 in library archives.