Anglo-Saxon history is generally described throughout three centuries: the fifth century, when Christian missionaries converted and partially colonized Ireland; the sixth century, when Anglo-Saxon invaders destroyed Celtic traditions in Wales and Britain; and the seventh century, when Gregory the Great sent missionaries to convert the Anglo-Saxons. In each case, native cultures were overthrown by an invading force, either political or military, while literature flourished and changed.
In Ireland, Christian missionaries of the fifth century encountered the survivors of the old pagan culture, including the file, poets who served as the kings' counselors. Many Irish legends and stories were already old, going back to the invasion of the Celts in the first centuries. The scribes in monasteries recorded the literature of Ireland in the sixth and seventh centuries, intending to mix it in with Christianity. Those monasteries were sacked during the Danish invasion in the ninth century.
Two centuries after the Anglo-Saxons first waged war against the Celtic inhabitants of Britain, their own culture came into contact with the Christian missionaries. A great amount of Old English literature was saved from destruction thanks to the efforts of some of the monasteries and people like Alfred the Great. King Alfred the Great ordered the literature to be restored in approximately A.D. 890, and generations of anonymous scribes added to the restoration until the 12th century. The original language was Old English, but entries in later centuries were in early forms of Middle English. (See: Beowulf, Battle of Brunanburh, Bede's Death Song, and http://ftp.std.com/obi/Anglo-Saxon/aspr/contents.html for a list of more in Old English)