R. Yosef Reinman (co-author of the excellent but controversial book, One People, Two Worlds) received a letter from an aspiring convert with a question about the name he wants to choose. R. Reinman asked how blog-readers would respond to the letter. The following is a brief excerpt and the response I would send.
I am thirty-eight and am going through a non-Orthodox conversion...
I need to pick a Hebrew name. Can you see any serious problem in these turbulent times with a convert to Judaism taking the name Yishmael ben Zion?
Dear X,
I thank you for your letter and commend you on the seriousness with which you treat every matter. I certainly do not want to insult you but I find it my duty to say that if the rabbi under whom you are studying for conversion has not properly explained how you will be viewed post-conversion by Orthodox Jews then you should investigate the matter. It is important that you understand the situation into which you are entering. Should you wish to further discuss this, please feel free to call me at 718-555-1212.
Regarding your question, the main issue with choosing the name Yishmael is whether will you be naming yourself after a wicked individual or group of people. Particularly since, as a convert, you will be know as "Yishmael ben Avraham," are you giving yourself the same name as our patriarch Avraham's wayward son? Additionally, as you pointed out, Muslims are frequently identified in midrashic literature as Yishmael. The answer, I believe, is that there is no technical problem with giving yourself this name. There are two reasons for this. First of all, we are taught that the original Yishmael repented at the end of his life.* Second, and perhaps more important, is what the great talmudic commentators whose teachings comprise the Tosafos taught us on this matter. They explained that one may not use the name of a wicked person only if there was never a righteous person with that name also.** Since there was a great sage of the Mishnah named Rabbi Yishmael, you may take his name and proudly wear it. If anyone asks from where you chose your name, you can explain the linguistic derivation as well as point out the great sage in whose path you have chosen to follow.
I wish you success on your journey and offer you my blessing, for what little it is worth, that you someday reach the level of spiritual succes of your namesake, Rabbi Yishmael.
With Torah blessings,***
"Simcha"
Note that I would not give mareh mekomos in this letter. The following are only for the sake of the blog.
* Rashi, Bereishis 15:15, 25:9.
** Tosafos, Yoma 38b sv. de-lo
*** I stole that sign-off from R. Yehuda Henkin