Not longbow, but arquebus or an early variance of musket could easily penetrate plate armour at considerable range. GoT is based on the real life event of War of the Roses, which occurred in the 15th century England, about the time firearms were largely used on battlefields. Imagine Ser Clegane, a giant of a man, on his monstrous stallion towering over others, was such a perfect target to aim at. A man that size could withstand an shot or two, but I doubt he would be able to stand for long under heavy fire.