Final answer:
The irony of Mason's fight to end slavery in the new colonies is that he owned slaves himself. He was afraid of freeing his slaves due to the economic implications it would have on the southern plantation economy.
Step-by-step explanation:
The irony of Mason's fight to end slavery in the new colonies is that he was himself a slave owner. While he believed in the ideals of liberty and equality, he was afraid of freeing his own slaves because of the economic implications it would have on the southern plantation economy. Many other southerners shared this fear, as the institution of slavery had become deeply ingrained in their society.