This is simply whisky and water, but as with the Japanese tea ceremony, observing the time and care taken over it making and the prolonged anticipation...
I am guessing based on Simon's results specifying a liquid, its probably referring not to a garnish of ume and shiso[also called perilla], but to a product like Umeshu[plum wine] brand Choya Umeshu Shiso. They also produce a Choya Umeshu Honey, which appears to be the product the "Risshun" refers to.