A warning: this answer refers to a regional dialect and definitely not to standard English. You're not likely to hear it as a learner as most people have manners and education enough not to speak in heavy dialect to outsiders, and it wouldn't be wise to try it out as it would probably be perceived as an error.
Colloquially, in parts of Ireland (including bits under the jurisdiction of the United Kingdom - for the nonce), it is definitely acceptable, even normal, to call 'regular men' sir.
In rural Ulster, one can call any man (and sometimes woman) of age sir and it carries with it no connotation of deference; it's used much as other regional Englishes use mate.
Due to the ethnic cleansing of the native Irish people and their culture from much of Ulster, and the settler colonisation of the north of Ireland with English-speaking Scottish lowlanders, to this day the English spoken in the north of Ireland is heavily influenced by the Scots dialect, and this sense of sir can be found in a Scots dictionary:
Scottish National Dictionary (1700–)
SIR, n. Also shir (Cai. 1891 D. Stephen Gleanings 82). Sc. usages:
As a common form of address between men of equal rank, esp. freq. among miners
(Slg., Fif., Clc., Ayr. 1970).
Used in addressing a lady. Obs. in Eng. exc. dial. since 17th c.
“The Highlanders use this term of respect indifferently to both
sexes” (Sc. 1904 E.D.D.).
I presume this usage is also still current in parts of Scotland too (especially as the reference for the first sense in the above dictionary entry is dated 1970), although I couldn't actually say so myself.
Other than this very limited dialectal sense, in standard British English, putting knights aside (who, in any case, deserve no deference, in my opinion!), sir implies deference, status and hierarchy: it's used by shop attendants to customers, by pupils to teachers etc.
See, for example, the Oxford Living Dictionaries definition, a British English dictionary:
sir (also Sir)
1 Used as a polite or respectful way of addressing a man, especially
one in a position of authority.