Why did Irish drop the O?

Why did Irish drop the O? In the 1600s, when English rule intensified, the prefixes O and Mac were widely dropped because it became extremely difficult to find work if you had an Irish sounding name. However, in the 1800s many families began reinstating the O and Mac prefixes.

In the 1600s, when English rule intensified, the prefixes O and Mac were widely dropped because it became extremely difficult to find work if you had an Irish sounding name. However, in the 1800s many families began reinstating the O and Mac prefixes.