The “Specials” were the Royal Irish Constabulary Reserve Forces and the Auxiliary Division, better known as the Black & Tans. The RIC Reserve Forces were recruited as a stopgap measure. Because of the short notice, their uniforms were a combination of khaki from the British Army and black (actually, very dark green) from the RIC. Hence the term, Black & Tans. This force was known for its brutality and attacks on civilians, quickly becoming the face of British oppression and injustice.
The real doctor’s name was Henry Johnston (not Johnson). The star he wears is a reference to the Williamite star used in Orange Order symbolism. The wearing of this star tells the audience that Dr. Johnson is a Unionist whose sympathies are against the Irish rebels.
This popular song was based on a real event. Willie Gillespie wrote the original lyrics in 1920, but the most popular version (and the one this story was based on) was by the Clancy’ Brothers. You can listen to it here.