Neither Douglas Bennett nor the authors of Cork Silver and Gold show this IW mark without pellet although Bennett does portray an IW mark without pellet in a serrated reserve. Jackson's Silver and Gold Marks shows an IW mark without pellet in a plain rectangular reserve, seen on a communion cup c1791 and a pair of two-handled cups, attributing it to John Warner, John Williams or James Warner.
You can forget John Williams as it is highly unlikely he ever had a mark on his own. Refer to my research on Carden Terry and John Williams here:viewtopic.php?f=56&t=40203
The style of the goblet, especially the circular beaded base, suggests c1780 so rules out John Warner's eldest son, James Warner. Refer to my Carden Terry Cork cream jug with similar circular beaded base here:viewtopic.php?f=56&t=39303
I've never come across that IW mark without pellet myself, but on the basis of what Jackson says, it seems entirely possible your goblet is by the Cork silversmith, John Warner, who worked from 1775-1810.