My understanding is that "Donegal" refers to tweed spun in the Irish region of that name, but since the plain-woven flecked pattern has become so iconic, it has become known as "Donegal" regardless of where it's from. The flecks are from the way the yarn itself is processed, so they can appear in weaves other than plain.
A tweed from Donegal can obviously be rightfully called a "Donegal" tweed. It may or may not have flecks. The flecks are from the way the yarn is processed. If a tweed NOT from Donegal has those flecks, and the flecks are caused by the same thing (nubs in the yarn itself), that tweed can also be called "Donegal." At least, that has been the practice. In other words, if you see a nubby flecked tweed in a book somewhere, it may be labeled "Donegal," yet not be from Donegal.
So, theoretically, you could have a Harris Donegal tweed (or Donegal Harris, whatever).
Harris though has very specific characteristics (in addition to where it was from). The yarn is very rough and fat, which makes the tweed extremely dense and scratchy, and the cloth thick and heavy. True harris is supposed to be hand woven on half width looms, using homespun yarn. Ie, it's basically a home made product almost totally lacking in polish.
Donegal yarn is much smaller, the weave is tighter, and the cloth is a lot thinner, less rustic.
I guess it comes down to what is causing the flecking. If the flecks are nubs in the yarn as a result of the wool being washed and felted before being spun into it, the tweed is then a Donegal-style tweed, regardless of where it's from. For example, I have a "Donegal" tweed jacket made from Porter & Harding cloth, but I have no idea where it's actually from. It's just called Donegal because of the sort of flecks it has.
So, it seems to me, if Harris tweed is defined by both the yarn type and place of origin, and such yarn cannot be spun to have such nubs, than a so-nubbed tweed from Harris is a Donegal tweed and not a Harris tweed, even if it is from Harris. If the yarn can be spun in Harris to qualify as Harris tweed yarn and have those nubs, then you have a Harris Donegal tweed.