As a Donegalwoman, there is no such thing as a Donegal sweater.
There is, however, such a thing as Donegal tweed. Donegal tweed is woven from wool, of course. But what is interesting about this wool happens during the spinning process. The fleece is dyed after scouring (cleaning and washing), then woollen-spun with minimal or no carding to remove short fibres. During the spinning process, these short fibres fall on the mill floor. After a few days or a week, the floor is swept, and the resulting semi-felted little bobbles (‘neps’) are added to the next batch of spinning. The yarn looks like this:
The neps stick to the yarn giving little pops of colour. A sweater knitted in this yarn could be called a Donegal sweater: