You're right that the CF gene is transmitted independently of genes for blond hair and blue eyes. There's no realtionship between these genes beyond their likelihood of appearing in certain populations. Celtic genes are very well travelled since various Celts occupied such a large and frequently conquered area. Almost everyone of Western European descent has at least a drop of Celt blood in his veins.
Remember also that the period of time in question is not a few hundred but many thousands of years. Because the CF gene has so many variations, it is thought to be at least 52,000 years old -- more than 10,000 years older than the earliest known European settlements. So the gene couldn't have spread from Ireland, where the first Celts arrived only a few thousand years ago. Rather, it came from Asia with the first European Caucasians. Them we can blame.
Remember also that the period of time in question is not a few hundred but many thousands of years. Because the CF gene has so many variations, it is thought to be at least 52,000 years old -- more than 10,000 years older than the earliest known European settlements. So the gene couldn't have spread from Ireland, where the first Celts arrived only a few thousand years ago. Rather, it came from Asia with the first European Caucasians. Them we can blame.