What does the name Eileen mean?

Answer 1

Josh is probably right about the immediate source -- "Evelyn" -- but it's Old Irish/GAELIC (not Greek!)
http://www.behindthename.com/php/view.php?name=eileen


But that doesn't help with the meaning! For that you have to go back a bit further.

There seem to be two basic possibilities and three suggested meanings:

1) Assuming it comes from Evelyn:
Evelyn is from the Old French name 'Aveline' (variant 'Eveline'), a diminutive of "Avila" which comes from "AVA".
http://www.behindthename.com/php/view.php?name=eibhli10n

So, what does "AVA" mean?

a) It MAY be related to Latin "Avis", "bird".

b) It is an Old German word which is part of several names, and may mean "desired"
http://www.behindthename.com/php/view.php?name=aveline
http://www.behindthename.com/php/view.php?name=ava-3


2) another possible derivation is from 'Helen', the Greek name, which may mean "torch" or "(the) bright (one)" (related to "Helios", meaning "sun")
http://www.behindthename.com/php/view.php?name=helen
http://www.aboutnames.ch/HWE.htm#gnEileen
http://www.aboutnames.ch/HWH.htm#gnHelene
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NOTE on sources:
I do NOT trust "baby name" sites and books -- they are very inconsistent, a mix of careful and shoddy scholarship, and unless you know the original language you can't tell what you're getting.

For this sort of question I HIGHLY recommend the sources I cited above, especially behindthename.com

If you are really into this sort of question, I recommend:

A software program called "Gwendolyn" which catalogs 8,000 personal names. (linked at behindthename.com)

*A Dictionary of First Names* edited by Patrick Hanks and Flavia Hodges (Oxford University Press, 1990)

Answer 2

This Irish or Gaelic name is a derivation of Helen, meaning light of the sun.

Answer 3

It is the Irish form of Evelyn. It is Celtic/Greek