The usual way to indicate the genitive of names in Modern Irish is simply to lenite the first consonant of the name, giving
mac Mhil, Mil's son or "son of Mil".
However, as Mick says, Old and Middle Irish worked differently and some names in particular have different vocative and genitive forms to what we expect nowadays.
Lenition wasn't written back then, so
Mac Mhil, even if the same, would have been written
Mac Mil.
Here's a transcript of the
Lebor Gabála Érenn manuscript(s):
http://www.ucc.ie/celt/published/G800011A/text001.html Translation here:
http://www.maryjones.us/ctexts/lebor1.html#14Looks like
Mac MIlid, is "a son, Mil". The genitive of
Milid appears to be
Miled, which would make "Son of Mil"
Mac Miled. Does that sound right?
I don't think the genitive of
Milid would be different for "sons of Mil" and "son of Mil", only the words "son" and "sons" should change, but I am not familiar enough with Old Irish to be sure.