sentences:  Secondly, they overhauled the IRA's structures, greatly reducing the numbers of volunteers who engaged in attacks and organising them into closed cells, or "active service units", so that the information any one IRA man would have about the organisation would be limited to five or six people.This process reduced the numbers of active IRA personnel in Belfast greatly.At its peak in the early 1970s, the Brigade had had up to 1,500 members.