I'm a grandson of a Mac Giolla. Not from my Irish lot but from my Manx lot:-
"MYLCHREEST and MYLECHREEST, contracted from MacGilchreest, a corruption of MacGiolla Chriosod, the son of Christ's servant.
Giolla, especially among the ancients, signified a youth, but now generally a servant, and hence it happened that families who were devoted to certain saints, took care to call their sons after them, prefixing the word Giolla, intimating that they were to be the servants or devotees of those saints.
Shortly after the introduction of Christianity, we meet many names of men formed by prefixing the word Giolla to the names of the celebrated saints of the first age of the Irish Church, as GIOLLA-AILBHE, GIOLLA-PHATRAIG, GIOLLA CHIASAIN . . .
And it will be found that there were very few saints of celebrity, from whose names those of men were not formed by the prefixing of Giolla . . .
This word was not only prefixed to the names of saints, but also to the name of God, Christ, the Trinity, the Virgin Mary, Some of the later forms of this name commencing with Mall, Maul, Molle, and Molly, would suggest a derivation from Maol, Mael, or Moel, which 'was anciently prefixed, like Giolla, to the names of saints, to form proper names of men, as MAOLCOLAIM, Maol-Seacnaill, which mean the servant or devotee of the Saints Columb and Secundinus. The word Mael means bald, shorn, or tonsured.
In the Isle of Mann, however, the earlier form is invariably MacGil, so it is probable that most of our Mylchreests are derived from MacGiolla. This name, and all those commencing with 'Myl' are purely Manx."
C