Well, it is a little difficult to explain for a French speaker but I hope I shall manage to do it. Old Gaelic (old Irish) had voiced and unvoiced stops which remained the same in modern Irish. In Scottish Gaelic voiced stops became unvoiced, between vowels and at the end of a word in some dialects, in all positions in other dialects, and unvoiced stops became pre-aspirated, for instance:
old Gaelic [d] > Scottish Gaelic [t]
old Gaelic [t] > Scottish Gaelic [ht]
Both changes were certainly simultaneous otherwise there would have been massive confusions between words. If it can help, it is an example of what André Martinet, a French linguist, called "chaîne de pulsion" (chain of pushing, if it is proper English): a phoneme takes the place of another one and "pushes" it to another position in the phonological system, so that confusions are avoided (for instance [d] becomes [t] and pushes [t] to [ht]).
I hope it is clear :)