The name MacGill appears in 18th century Argyll in forms such
as McGeyll / McIgeyll / McGeill / MacIgheil. In a number of entries in the
Inverary Sheriff Court "Register of Bonds" individuals bearing these
names are also given the name McIvoill, which is a well known phonetic form of
Mc'illemhaoil / MacGhillemhaoil - one of the two Gaelic originals from which the
name MacMillan is derived. In other words the "Gill" of MacGill stands
for "Mhaoil", as is most apparent in other transitional forms found in
the 16th, 17th and 18th centuries, such as McVeil / McIveil / McIyell.
In the 15th century the Barons MacIyell / McGeill were lairds of Barrichbeyan, a
property that was originally part of the estate of Craignish; and the traditions
of the MacCouls of Craignish suggest that these two families shared a common
origin (MacCoul is a phonetic version of MacDhugaill, but this clan did not
belong to the same kindred as their neighbours the MacDougalls of Lorn - and
probably in fact took their name from Dugald mac Malcolm mhic Mhaolain).
In later centuries there were quite a few McGills in Kintyre and local traditions there confirmed that they were in origin McMillans - as reported by Andrew McKerral in his 1948 book "Kintyre in the Seventeenth Century". The most striking case yet found of the variety of names that eventually became - in some cases - "MacGill", is a drover and changekeeper in Kilmartin, Glassary, in the 1760s called Duncan, whose many entries include a surname given variously as McIgheill, McGheill, McIyeall, McYeill, and Bell - as well as McIlvoill; i.e. MacMillan.
For a more detailed account see
"The Surnames M'Gill and Bell" by Graeme M. Mackenzie in The
Scottish Genealogist, Vol. LII, No. 4, December 2005.