Lower, Mark A (1860) Patronymica Britannica: a dictionary of the family names of the United Kingdom. London: J.R. Smith. Public Domain.
We don't have any stats on how common this name is. This is probably because it's very rare in the UK.
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The By surname in historical dictionaries
Patronymica Britannica (1860)
KIR BY. (In charters, Cherchebeius, and. originally written Kirldjy). Parishes, &c., in cos. Essex, York, A\''arwick, and Norfolk, Places in cos. Lancaster, York, Nottingham, Lincoln, Leicester, and Westmoreland, still retain the form Kirkby, which is also a surname.