Romulan ale was brewed in wooden barrels from several grains, including satla and kheh. The inhabitants of Virinat used khellid nectar as an additive. (The Romulan Way, Lch'R: "A Day on the Farm", "An Afternoon in the Hive")
Romulan ale was roughly equivalent in alcohol content to an Earth wine or particularly strong ale. It was difficult for even Romulans to drink due to being harsh to the throat (a common joke said that the Romulans drank it to prove that they could), although the better brewers such as t'Thavrau Wineries were able to moderate the burn and focus on the flavor. (The Romulan Way, Lch'R: "An Afternoon in the Hive", TWR: "Somebody Else's Wedding")
Romulans drank it with meals or as an aperitif. One common toast was, "Ei e'hraaintuh na'hwiufvteh, emeihet'!"
The Romulan Star Navy and Romulan Republican Force both exercised sharp limits on its consumption aboard ship and did not allow it during yellow alert or above. This stemmed from an infamous incident in 2088 when the IRW Avatar of Tomed was captured by a Klingon bird-of-prey because most of the crew were too drunk to fight.
Romulan ale was technically illegal in Federation space under long-term trade embargos against the Romulan Star Empire, but the Starfleet Criminal Investigative Service tended to turn a blind eye to its consumption, endemic among Starfleet personnel as a test of mettle among other things.
The embargo was briefly lifted during the Romulans' membership in the Federation Alliance during the Dominion War, but by 2379 it was back in place. (DS9: "Inter Arma Enim Silent Leges", Star Trek Nemesis)
The virtual collapse of the Empire after the Hobus supernova in 2387 rendered the embargo largely obsolete, and while importation from Imperial planets remained prohibited, importation from independent and eventually Romulan Republican planets was affirmed as legal by the Federation 27th Circuit Court of Appeals in 2398.