Ryncol is an extremely potent liquor created by the krogan. Its exact composition is unknown, but it is clearly a very strong drink, given that it is created by and for the krogan. Grunt comments that ryncol "hits aliens like ground glass".
When Commander Shepard visits the Dark Star Lounge and asks the bartender for five consecutive drinks, the Commander will be given a glass of ryncol. The bartender jokes that Shepard will "set off radiological alarms" after they drink it. During the mission N7: Mining the Canyon, a datapad contains a complaint made by a pirate about how the YMIR Mech he purchased "leaks [fuel] like a volus after a mug of ryncol".
Ryncol is green and is apparently a cocktail; the turian bartender in the Dark Star Lounge creates the glass served to Shepard by mixing separate blue and green components together. As per his prediction, it does indeed knock Shepard out, leading to a short sequence where the Commander groggily wakes up on the floor of the Dark Star's bathroom. While at the Silversun Strip, Grunt will mention that ryncol is also flammable, and in fact used it as a Molotov Cocktail by merely throwing a regular bottle at a C-Sec aircar.