Etymology

Formed within English on are (adv.), a variant of certainly native ere (< OE ǣr (comp.)), usually explained as a loan from ON; thus OED3 suggests ‘possibly after early Scandinavian’, comparing OIcel árliga. The OIcel adv. was most often used to mean ‘annually’ in prose, but in poetry was used in the same way as árla with the sense ‘early’.
 

PGmc Ancestor

*air-

Proposed ON Etymon (OIcel representative)

ár 'early', cp. árliga ‘annually’
(ONP ár (4) (adv.), cp. árliga (adv.))

Other Scandinavian Reflexes

Far ári, Icel ár, Norw år

OE Cognate

cp. ǣr (comp.) 'before, earlier'

Phonological and morphological markers

Summary category

CC1c

Attestation

This form of the adv. first appears early in the 13c and variants in <a> or <o> are attested most frequently from the N/EM. See also are (adv.). Note that the variant with native-derived <e> occurs at WA 3444. The positive degree ār is attested in Nhb OE only, on its own and in various complexes, eg. ārlic 'early' (see further SPS 73).

Occurrences in the Gersum Corpus

WA 350, 351

TPD emend the instance at WA 351 to *radly ‘soon’ for metrical reasons.

Bibliography

MED ē̆rlī (adv.)[ https://quod.lib.umich.edu/cgi/m/mec/med-idx?type=id&id=MED14339&egs=all&egdisplay=open], OED3 early (adv.)[ http://www.oed.com/view/Entry/58988?isAdvanced=false&result=2&rskey=NR5sbY&], HTOED ; see further are (adv.).