"Says Lloyd George to MacPherson, I give you the sack To uphold law and order you haven't the knack." The English create the Black and Tan army, which commits atrocities, but the Irish vow they will defeat the English
By 1920, Irish terrorism had clearly reached the point where the normal authorities could not control it.
This was especially true since the regular members of the Royal Irish Constabulary (RIC) were losing their enthusiasm. By this time, though only a few dozen had been killed, their morale was falling; by late 1920, roughly 10% had resigned (see Robert Kee, _Ourselves Alone_, being Volume III of _The Green Flag_, p. 96), and the rest had perhaps lost their edge. The British saw a need for more replacements than could possibly be raised in Ireland itself (Kee, p. 97, says that they eventually recruited some 7000 new police), and started importing potential police from Britain itself. British Prime Minister David Lloyd George therefore recruited a special auxiliary force (known as the Black and Tans) to try to restore order.
The Black and Tans are often called the dregs of British society. This is at best an exaggeration. It is true that most were unemployed -- but this is hardly their fault; they were World War I veterans, often taken into the army as soon as they finished school, and then returned home to an England where all the jobs were filled.
As Calton Younger puts it (_Ireland's Civil War_, p. 105), "They were not the dregs of English jails, as Irishmen have so often alleged, but bored, unsettled, often workless ex-soldiers, young men whose ordinary pity and honour had been dried up by their long and merciless ordeal in the trenches." One might add that, having been so long under discipline, it took only a few really bad apples to lead them to brutality.
Their black and tan uniform was largely an accident; as there were not enough Royal Irish Constabulary uniforms available, the Black and Tans received a mixture of oddments.
The Irish correctly accuse the Black and Tans of atrocities -- the British (exhausted by World War I) had little choice but to fight terror with terror. The Black and Tans were the worst mostly because they had no experience of the Irish except during the terrorism. With their comrades being attacked from hiding with terrorist weapons, they took revenge where they could -- even if it meant random revenge which hurt their cause more than it helped.
The British did not entirely ignore the Black and Tan problem; Kee reports (p. 117) that 218 of them were dismissed as unsuitable, and a few dozen were subjected to prosecution for their behavior. This did little to control the problem. Technically, the Black and Tans were keeping Ireland in British hands; Richard Mulcahy, the Irish Chief of Staff, who was one of those chiefly responsible for fighting them, observed that, for all the deaths, the Irish rebels had never managed to drive the English out of anything more significant than "a fairly good-sized police barracks? (see Kee, p. 145). But military control is not peace. (Just ask any citizen of Iraq.)
The results were intolerable. Both sides agreed to a truce in 1921, with elections to follow in Ulster and the rest of Ireland. As it proved, Sinn Fein won overwhelmingly in Ireland and Unionist (i.e. pro-British) parties almost as completely in Ulster. The path to Irish independence was at last clear -- as long as the country was willing to accept partition.
The MacPherson of the song is Sir Ian MacPherson, Lloyd George's Irish Minister, who believed in Home Rule and, although he fought to keep order, was not strict enough for the Prime Minister.
Macready is Major General Sir Nevil Macready, Commander in Chief of the British forces in Ireland. George Dangerfield describes him as impartial in the Irish struggles; "he disliked both sides" (nationalists and Ulstermen; see Dangerfield, _The Damnable Question_, p. 319; also p. 110, where it is said he had "no sympathy for either Nationalists or Orangemen"). - RBW