It is possible that this was a son of Donnall mac Uilleim.
Balfour Paul does not name Malcolm's mother as Margaret, just that Madach had two sons and Harold was born in 1134, just after the second marriage.
Sources
↑Paul, James Balfour, Sir, 1846-1931 : The Scots Peerage : Founded On Wood's Ed. Of Sir Robert Douglas's Peerage Of Scotland; Containing An Historical And Genealogical Account Of The Nobility Of That Kingdom : Free Download, Borrow, And Streaming : Internet Archive". 2020. Internet Archive.>Malcolm, 2nd Earl, Vol I, pps 416-8
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Source: Charles Cawley, "Scotland, Mormaers, Earls, Lords: Chapter 2 Atholl: Earls of Atholl", Medieval Lands: A prosopography of medieval European noble and royal families, v3.2 Updated 25 April 2017, Foundation for Medieval Genealogy (FMG), (http://fmg.ac/Projects/MedLands/SCOTTISH%20NOBILITY.htm#Malcolm2Atholldied11861198A accessed 4 October 2020).
Thankyou!