Who were they?
In 18th century Ireland, the Whiteboys were a breakaway sect of agrarian rebels, usually catholic peasants, who rebelled against the state practice of tithe collection ("God tax"), high rent and the enclosure of previously common farming land by protestant landowners. They were origrinally run by a man known only as "Captain Midnight".

They always claimed not to be driven by sectarian motivation, yet the political climate between Ireland and Great Britain at the time prompted the English government to send an army over to Ireland to annihilate this small band of mostly unarmed repressed farmers.

They did not succeed.

Later attempts to curb these movements spawned the Whiteboy act in 1796 and the Insurrection act in 1816 resulting in martial law in certain areas and the effective removal of "habeas corpus". This led to many rushed convictions and death sentences of rural Irish "upstarts", both innocent and guilty.

"If the Insurrection Act derives its forces from the principles of coercion and terror, it has suspended a tyranny which carried both to the utmost extremity of barbarous and relentless cruelty; which had become irresistible by the ordinary powers of law and which unresisted, must have reduced Ireland to incapacity."

Whiteboys and similar movements in Ireland:
Rockites led by "Captain Rock"
Rightboys
Molly Maguires
Houghers (who specialized in cutting the hamstrings/hocks of farm animals)
The Duagh Boys
Whitefeet
Terry Alts
Hearts of Steel (Steelboys)
Hearts of Oak (Oak Boys, active in Ulster)
Greenboys
Peep of Day Boys
Thrashers
Defenders
Shanavats
Caravats
Lady Clares

What did they do?
They were a well-organised underground movement who operated mostly at night wearing white smocks as a disguise. The height of their mischief would have been the burning of a few fences or killing some farm animals, but occasionally the violence would descend into murder. With the support of the entire rural community in Munster and Ulster, they remained an elusive yet long-lived movement throughout the 18th and early 19th century.

How were they stopped?
The group were more or less disbanded when the Famine hit Ireland decimating the population of the country forever.


sources: Irish Midlands Ancestry