Hi! I've answered this previously. If you have any further questions, let me know.

Ireland's supposed contribution to modern terrorism comes in the form of the "cell system". In 1858 the Irish Republican Brotherhood was formed in Ireland by James Stephens. It was a secret oath bound society and for much of its early existence it had no formal name, being known only as "The Organisation". Members sworn into the IRB joined "circles", groups of nine individuals headed by a "centre". The circle centre would communicate with similar sized units at regional and national level. In the event of an informer infiltrating a circle or of a member being arrested, damage was limited to that circle as only a single person had contact with members outside of it. This made it difficult for British authorities to deal a decisive blow against the organisation and allowed them to carry out a protracted bombing campaign in the 1880's.

This system allowed the IRB to function from 1858 until 1916 when they staged an unsuccessful rising in Dublin, following which many members of the Supreme Council, the organisations governing body, were executed or arrested. Though it never regained its full strength it remained an influence in Irish military and political affairs until its dissolution in 1924.

As u/Bernardito mentions in his answer a number of individuals were influenced by Michael Collins, who was President of the Supreme Council from 1920 to his death in 1922. Tim Pat Coogan says that Mao Zedong had studied Collins tactics. It could be argued that the circle system developed by James Stephens has been adopted by a number of modern terrorist groups such as Al Qaeda. The Provisional Irish Republican Army adopted this system in the late 1970's with Active Service Units of 5 volunteers and smaller. This allowed them to act independently and theoretically limited the effects of informers, though some modern accounts show the organisation to have been heavily infiltrated. The PIRA were connected to other terrorist organisations and trained in Libya with a number of Middle Eastern groups.

Stephens adopted the cell system due to his study of Irish history which had shown the damage done by informers to previous organisation and rebellions. It is possible that this system was developed independently by other groups out of necessity as it was the most successful way for them to operate. I don't know if any groups used a cell system before the IRB but this is what is usually meant by Ireland inventing modern terrorist techniques.

Sources:

Tim Pat Coogan - "Michael Collins"

Owen McGee - "The IRB"

Ed Moloney - "A Secret History of the IRA"

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Edit: Originally said that Tim Pat Coogan claimed the Viet Cong was inspired by Collins, I was mistaken in this, his introduction actually claims Mao Zedong studied his tactics.