Complex sentence: All these groups claim sole legitamist descent from the original Irish Volunteers and Irish Republican Army, and thus also claim the sole right to use the name "Óglaigh na hÉireann".
Split into simpler sentences: All these groups claim to be the sole legitimate descendant of the original Irish Volunteers and Irish Republican Army. Thus they also claim the sole right to use the name "Óglaigh na hÉireann".