The following excerpts are from   The Surnames of Scotland    Their origin, meanings, and history, by George Black, PHD, New York Public Library, 1946. They were collected for the book from articles which originally appeared in the Bulletin of the New York Public Library, August 1943 through September 1946.

MACLAE, MACLAY, MACCLAY, MACLEA, MACLEAY. These names are commonly but incorrectly explained as from G. Mac an leigh. This is merely popular etymology due to the belief that the name means 'son of the physician,' and that the Macleays of western Sutherland are descended from Ferchard Leche, who had a grant of lands in Assynt, 1386. The full form of the name is MacDhunnshleibhe, 'son of Donnshleibhe,' as explained under DUNSLEVE, q.v. In a royal commission of 1498 King James III directed certain persons to distrain the lands and goods of Kenyoch M'Conleif and Donald M'Conleif, co-raiders with Chisholm of Comar of the lands of Huchone Ros of Kilrawok (OPS., II, p. 527). Kenzoch or Kennitus M'Coleif held the king's lands of Cumree ( Comrie ) in Strathconon in 1504 (ER., XII, p. 663), behind Tor Achilty, where tradition placed the seat of the clan. John M'Ewin V' Dunslef appears at Sonnachan, Argyllshire, in 1502 (Notes and queries, 11 July, 1931, p. 21), and Finla Makgillecallum Makcolluf and John Roy M'Culloiff were tenants under Stewart of Appin in 1509 (Stewarts of Appin, p. 193). In 1518 the "clane McDowleanis" (an error for M'Donvleavis ) gave their bond of manrent to Sir John Campbell of Caldor (Cauwdor, p. 129). The names given in the bond are: Duncan Brec McDunlane, and Jhone Mcdoulane 'his broder.' Dunslane McNeill is one of the parties to the bond, and in all three instances -laue has been misread -lane. John McYndayn McCollef is mentioned in 1519 (ibid., p. 131). Odoni Makdouill Makdunlane [i.e. Makdunlaue] had a remission in 1524, and a son of Duncan M'Dunlewe was appointed minister of the two parishes of Killespic-Kerrill and Kilmaronock in 1541 (HMC., 2 Rep., p. 193). The northern Macleays were known to Sir Robert Gordon as the clan Leajwe; while thereafter they are known in record as Makley (in Alness 1651), M'Ley (in Contin 1677). Macleays were numerous in early times in Easter and Wester Ross and in Argyll. Of the southern Macleays was Jo. M'Ey V'Dunlaif in North Argyll, 1570 (Cawdor, p. 177). A cautioner was found for John McClay in Ayrshire, 1584 ( RPC., III, p. 714), and Donald M'Collea was one of Dunolly's men in 1588 ( RPC., IV, p. 265), and Duncan M'Collea appears in same year (ibid., p. 333). Patrik M'Conlea in Finran was fined for reset of members of Clan Gregor in 1613 (ibid., xn, p. 631), and Donald M'Onleif was servitor to Allan Cameron of Locheil in same year (RMS. VII, 871) . Donald M'Clae was burgess of Glasgow by purchase, 1617 (Burgesses), and Duncan Dow McDonochie Brec alias McDonnslae and John McEane Dow VcDonochie Brec alias McDonnslae were charged with assault and cattle-stealing, 1623 (RPC., XIII, p. 270-271). Mc onlea is a common record spelling of the name, and its derivation is clearly shown by the name of Dounslea Mconlea, tenant of Schewnay, 1669 (HP., IV, p. 222). Donald roy M'Onlay appears in Lunga, 1633 (Notes and queries, 18 July, 1931, p. 44). Findlay McClay and Malcolm McClay appear in Drakies in 1646 (More Culloden Papers, p. 50), and Dunsla M'ein Vc onlea and Iain M'onlea, his brother, were among those massacred at Dunaverty, 1649 ( HP., II, p. 257). Duncan Mc onlea was bailie of Rothesay, 1662 (ibid., III, p. 12), and another Duncan M'Onlea was burgess of Paisley, 1667 (Isles). Iain Mc onlew was tenant of Torisay, Mull, 1669 (HP., IV, p. 223), and Duncan M'Onlea, Duncan M'Dunslea, and Neill M'Dimslea ( an error for Dunslea) appear in Islay, 1686 (Bk. Islay, p. 508, 511). William Livingston, the Islay bard, always wrote his name in Gaelic M'Dhunleibhe, and Dr. David Livingstone, the African traveller, was a Macdonleavy of Ulva. Allan Dall gives another popular etymology of the name, which he makes to mean 'son of John the grey-haired,'