Sing is native, Old English singan, and it is one of the best surviving examples of a Germanic strong verb — a verb that marks its past tense by changing the vowel in the middle rather than adding an ending. Sing, sang, sung is not an irregularity that English failed to tidy up; it is the older system, inherited from Proto-Germanic and visible across the family in German singen, sang, gesungen.
That pattern is a reliable sign of native vocabulary. Borrowed verbs almost never join it: nobody says "I have sang" for a French or Latin verb, because English forms those with -ed. Drive, drove, driven and ride, rode, ridden work the same way. The verb was used in late Old English of birds and wolves as well as people, and its origins beyond Germanic are obscure — there may be no certain relatives outside the family at all.
sing (v.) Middle English singen , from Old English singan "to chant, sing," especially in joy or merriment; "celebrate, or tell in song" (class III strong verb; past tense sang , past participle sungen ), from Proto-Germanic *sengwan (source also of Old Saxon singan , Old Frisian sionga , Middle Dutch singhen , Dutch zingen , Old High German singan , German singen , Gothic siggwan , Old Norse syngva , Swedish sjunga ), from PIE root *sengwh- "to sing, make an incantation." Also used in late Old English of birds and wolves, and sometimes in Middle English also "play on a musical instrument." There are said to be no related forms in other languages, unless perhaps it is connected to Greek omphe "voice" (especially of a god), "oracle;" and Welsh dehongli "explain, interpret." The typical Indo-European root for "to sing" is represented by Latin canere (see chant (v.)). Other words meaning "sing" derive from roots meaning "cry, shout," but Irish gaibim is literally "take, seize," with sense evolution via "take up" a song or melody. The sense of "utter enthusiastically" (of praises, etc.) is from 1560s. The criminal slang sense of "to confess to authorities" is attested as early as 1610s, but modern use probably is a fresh formation early 20c. To sing for one's supper , implying lack of funds, is by 1745. Every child should be taught, from its youth, to govern its voice discreetly and dexterously, as it does its hands ; and not to be able to sing should be more disgraceful than not being able to read or write. For it is quite possible to lead a virtuous and happy life without books, or ink ; but not without wishing to sing, when we are happy ; nor without meeting with continual occasions when our song, if right, would be a kind service to others. [Ruskin, "Rock Honeycomb"] sing (n.) "an act of singing, an entertainment of song," especially collective, 1850, from sing (v.). also from 1850