One reason was that it took a barmaid a long time to serve as she had first to put in the black liquer and then with adifferent nozzle put in the white liquer
Guiness had working for them a mathematician called Mr Sheridan. They gave him this problem He designed a new nozzle that automatically supplied the correct amount of each liquid in the correct sequence. The serving time was dramatically shortened and the sales improved.
Guinness called the liquer after Mr Sheridan