The Old English form is "Rægenhere" (attested for example in the name of the son of king Rædwald of East-Anglia).
[1] The name was variously Latinized as Raganarius, Reginarius, Ragenarius, Raginerus, Ragnerus, Reginherus.
It enjoyed a revival in the late 19th and early 20th century, in connection with national romanticism in Scandinavia.
The name is now current as Ragnar in Iceland, Norway, Sweden and The Faroe Islands and as Ragner in Denmark.
[2] In Iceland, the name remains popular, recorded at rank 21 (given to 0.76% of newly born boys) as of 2014.