In 2005, they formally renounced violence and instructed its active units and volunteers to cease all activities and "assist the development of purely political and democratic programs through exclusively peaceful means.

Irish Republican Army (IRA).

Explanation:

An Irish Republican Army (IRA) was established in 1919 with the purpose to gain independence from the British in Northern Ireland using armed forces. It is a paramilitary force that aims at establishing a Republic in Northern Ireland. The organization was supported by Sinn Fein at the political level.

In 1969, the IRA split into two groups namely Officials and Provisionals. Officials sought for peaceful ways to attain independence whereas Provisionals sought the ways of violence using armed forces.

After much violence and many deaths, on 28th July 2005, IRA officially announced that it will follow the ways of peace to achieve its goal and cease from the violent activities.

So, it is the Irish Republican Army that officially renounced violence in 2005.

i'll just take the points xd