Giving directions from the Cambridge Theatre is a bit tricky because it's at Seven Dials which, perhaps unsurprisingly, has 7 roads coming off it! It's easy to walk down the wrong street!
See the picture of the theatre, here, and imagine that you're leaving it:
http://www.thisistheatre.com/londontheatre/cam bridgetheatre.html
Don't take the road, immediately on your left, which runs alongside the theatre but
take the the next nearest road on your left-hand side. This is
Monmouth Street, which is the principal road through Seven Dials.
The first turning on your right will be Tower Street. Carry on past this, to the next junction. At this point, both Litchfield Street and West Street join Monmouth Street simultaneously. West Street (where The Ivy is located) is the right-hand one of the pair.
Chris