This is my first post on this board. My last name is Killelea, and the earliest spellings vary a lot (These are from Queen Elizabeth's pardons):

McGilaleghe
McGillalea
McGilla Leigh
McGilla Ley
McGilleleigh
McGilleley

But those are English translations of course. The MacLysaght book has Mac Giolla Léith, and Patrick Woulfe's book has Mac Giolla Liath.

Which spelling is probably right, MacLysaght or Woulfe? Are the pronounced the same? Do they both mean "son of the servant of the grey"? What might that mean anyhow?

Could it possibly be Mac Giolla Léigh? That wouldn't make any more sense (léigh is the verb "read"), but it would fit better with the English spelling, and I think it's pronounced the same.

Any clues appreciated!

I've got to say firstly that I am no expert on surnames. I do know that you will often find different anglecised forms of the same Irish name, depending where in the country, and perhaps when, the anglecisation occurred. Also, different Irish language names have been anglecised to the same 'English' name.

In your case you appear to have done research and narrowed it down to two forms. Both are very much related. The genitive form of liath is léith so what you have there is a grammatical difference only. Why the difference? I don't know! There may have been resistance to using the genitive of adjectives in one part of the country while it was used in another, and if this is so you may need to do some more research to see which version is appropriate for you. Certainly wait for other comments as I know little or nothing about this topic in reality.

Mac = son
Giolla = attendant / man-servant amongst other meanings
Liath (léith) = grey (= gray)

I see from a quick google search that there is a professor at UC Dublin named Caoimhín Mac Giolla Léith. He has a lot of hits on google, so he should be relatively easy to contact. Since he uses the Irish form of his name, he may know some of its history, so you might try contacting him.

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I'm not a native (or entirely fluent) speaker, so be sure to wait for confirmations/corrections, especially for tattoos.

Thanks for the tips! I will contact Caoimhín Mac Giolla Léith and if he answers I will post it here.