long ago vs long since

long ago and long since both are adverbs.

Word	Noun	Adjective	Verb	Adverb
long ago	No	No	No	Yes
long since	No	No	No	Yes

As adverbs, long ago and long since are synonyms defined as:

Other synonyms of long ago include lang syne, langsyne.

long ago (adverb)	long since (adverb)
of the distant or comparatively distant past	of the distant or comparatively distant past