The Irish soldiers were to join the German mercenaries already in training in Rio de Janeiro to fight alongside the Brazilian Army. Advertisements were run in local newspapers, and notices were posted on numerous church doors, mainly in County Cork and in County Waterford. The Colonel promised free passage, free land, six shillings per day, and military training as local militia only.
Almost 3,000 mostly poor and illiterate people quickly volunteered to make the long and dangerous sea voyage. Some sold what little they owned to buy farm implements for their new life in Brazil. Most apparently did not realize that they had been recruited to fight as mercenaries. 2,700 people actually showed up on sailing day, and boarded the nine ships anchored in Cork Harbor.
Among the volunteers were the John Clancy family, from near Waterford in Ireland. The family consisted of John Clancy and his wife, Mary (or was it Elizabeth) Clancy, nee Ahearn, along with their two daughters, Nancy and Ellen, and a son, name unknown. It was from newspaper interviews with Nancy Clancy on her birthdays in her latter years (she lived to be 95), that the hardships of their voyage came to light. While at sea, the young Clancy son died of yellow fever. His body was used as bait to catch the shark that had been following the ship. The boy was then removed from the shark's stomach and given a proper Christian burial. Afterwards, the shark was divided up among the hungry passengers. Then their ship was wrecked off Tenerife with the loss of more than half of the passengers. The replacement ship had to make an emergency stop on an island off the coast of South America, where only the hospitality of the local natives saved them from starvation. The replacement ship reached Rio de Janeiro in late January 1828, when most of the other ships arrived.
Rio de Janeiro's black slaves and the Irish did not get along. Taunts of 'white slaves' when the Irish first landed escalated into individual fights, then large scale brawls, and finally, into murders by roving bands on both sides in the dark streets.
Unrest among both the Irish and the German mercenaries due to rough treatment, non-payment of wages, general misery, and rumors of going into battle soon, grew. The similarly recruited (and deceived) German mercenary soldiers started the Great Mercenary Revolt on 9 June, 1828. When one of their number received many lashes for a minor infraction, the Germans freed their comrade, and attacked the hated officer, who fled for his life. Word of the German revolt quickly reached the Irish, and about 200 Irish joined. Weapons and liquor were seized. Irish sources state that the homes of a few hated officers were looted and burned by marauding bands. Brazilian sources record that whole blocks of downtown Rio de Janeiro were razed.
By the second day, it was realised that the available Brazilian troops in Rio de Janeiro were insufficient to quell the armed and drunk mobs. Black slaves, who needed no coaxing, and other citizens, were given arms and sent against the mercenaries. The Irish and Germans were slowly pushed from the streets and back into their barracks, their best defensive positions.
The emperor requested and received help from marines aboard British and French ships in the harbor. Not wanting to fight against them, many of the rebel barracks surrendered on the third day. The final barracks building was only taken by storm on the fourth morning with very heavy casualties on both sides.
Others were sent to North America. The John Clancy family sailed directly from Rio de Janeiro to Portland, Maine in America. On the way, they were shipwrecked and lost another child. Another ship from Rio de Janeiro landed more than 200 Irish passengers at St. John in New Brunswick, Canada, and 32 of them made their way to St. Andrews in New Brunswick, Canada. Some arrived with little more than the clothes on their backs, while others had money to spend.
The mutiny virtually destroyed two of Dom Pedro's supposed best units and ended his hopes for a land victory to augment his successful naval blockade of Argentina. Brazil and Argentina both agreed to give up their stalemated war. Dom Pedro ratified the peace treaty on 28 August 1828, and Uruguay became an independent buffer state between the two South American giants.