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I have never, ever met any woman born in Ireland, or to Irish parents, called Cassidy or Shannon as a first name!
Heh, I think in those cases they may be referring to Irish-American names. I only know a few Cassidys (both first and last name) but I know many women with Shannon as a first name and one who had it as a last name.

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Numerology works, in theory, by counting up the letters in a person's name (first, middle, last, etc.) and determining whether the sum is a "lucky" number or not.
That's one form of numerology. Another is to assign each letter a number and then add the numbers together. Read more about that here.

So, for example:

Clark Kent
33192 2552

Then you add the numbers in the first name and last.

(Clark)
3+3=6 6+1=7 7+9=16 16+2=18

If the number comes out in the double digits you add each digit (in this case 1+8=9) so you get a single digit.

(Kent)
2+5=7 7+5=12 12+2=14 1+4=5

Finally, 9+5=14 which we reduce to a 5 .

Whew.

Back to the question, I usually pick a name based on what seems to fit for the character. If they are related to canon characters that will have an impact on the name I select.

Sometimes, if I can't decide between names I will look at baby books or other sources to see what the meaning and history is behind a name.

Also, if there are certain names or letters that have meaning in the story I'm telling or the universe it's set in (if it's fanfic) then that will affect it as well.

"Let us remember that there is a creative force in this universe, working to pull down the gigantic mountains of evil, a power that is able to make a way out of no way and trasform dark yesterdays into bright tomorrows. Let us realize the arc of the moral universe is long, but it bends toward justice."
-- Martin Luther King Jr.