http://en.wikipedia.org/wiki/Mansfield_Smith-Cumming
When Britain's Government Committee on Intelligence decided to slash Kell's Special Intelligence Bureau budget and staff and subordinate MI5 under a new Home Office Civil Intelligence Directorate led by Special Branch's Sir Basil Thomson in January 1919, the powerful MI5/Special Branch partnership that admirably managed counterintelligence and subversives during the war was suddenly thrown into disarray. These bureaucratic intrigues happened at the very moment that the Irish abstentionist party, Sinn Féin, and the Irish Republican Army (IRA) were launching their own independence campaign.
With MI5 reduced to a skeleton staff of just 28 officers and relegated to the sidelines, and with Thomson unable to contain or penetrate the revitalized IRA with a series of clumsy and hastily organized police intelligence operations, it fell to Smith-Cumming who was then head of SIS (MI6) to organize a new espionage unit in Ireland, based on continental lines and called the Dublin District Special Branch, in mid-1920.
The DDSB consisted of some 20 line officers drawn from the regular army and trained by Smith-Cumming's department in London. Beyond that, however, Smith-Cumming began importing some of his own veteran case officers into Ireland from Egypt, Palestine and India, while Basil Thomson organized a special unit consisting of 60 hastily vetted ethnically Irish street agents managed via impersonal communications from Scotland Yard in London.