Complex sentence: McKevitt was a senior longtime member of the Provisional IRA and served as the organisation's Quartermaster General, a role which gave him unique personal knowledge of the whereabouts of its arm dumps and access to them.
Split into simpler sentences: McKevitt was a senior longtime member of the Provisional IRA. He served as Quartermaster-General of the PIRA, a role which gave him unique personal knowledge of the whereabouts of its arm dumps and access to them.