The Hebrew word for the act performed at a circumcision is "לימול". The root appears to be נ.מ.ל. Following the exceptional rules for roots whose first letter is נ, the causative participle and nominative should be "ממיל" (with the second מ דגושה). Why then is the universal word for one who performs the act of circumcision "מוהל" as if the root is י.ה.ל (e.g.), which can't possibly be the case?
The root is מול, sometimes considered a biliteral root, מל. Compare קם.
There are cases in the Semitic languages where a full consonant appears in the middle of such roots to "fill them out" to the usual triliteral form. Compare Aramaic רהט "run" (Hebrew רץ).
The Talmud Yerushalmi in Mesechta Yevamos דף מג,ב פרק ח הלכה א uses the word as follows כההיא דאמר רבי הילא בשם רבי יסא הלוקח עבדים ערלים מן הגוי על מנת למוהלן. I guess that is the source of the ה
I believe you have the wrong root: http://ericlevy.com/Revel/BDB/BDB/14/num108.html#t59 what נ.מ.ל gives. For the root see: http://ericlevy.com/Revel/BDB/BDB/13/mem32.html#t18