As well as the wrong pronunciation of simplí, isn't madra a masculine noun? If so, shouldn't it be "Is madra é" ?
If the dog is female, í is right. But you're right, madra is still grammatically masculine, which is why simplí is not mutated to shimplí.
For gendered people, animals, and sometimes means of transportation, the social gender wins, not the grammatical one.
For instance, cailín is a masculine word...