When referring to two or more clusters of ivy, is it ivy, ivies, or something else entirely?
Ivy is similar to grass (and water). There is technically a plural, but in practice it is almost always treated as a quantity noun (a word that is inherently plural). This is because it grows together in big intertwined clumps, so it is nearly impossible without manually uprooting to tell it which ivy leaves belong to different plants and which to the same plant.
So the only time you'd really use the plural would be in talking about multiple different patches of it, or in talking about all the different varieties of it. eg: "My yard is overrun with ivies" might indicate that I have multiple different species of the plant. If I have just one big patch growing, I'd say, "My yard is overrun with ivy."
The plural of "ivy" is "ivies"
T.E.D.'s answer has covered the uses of ivy and ivies, but if you actually need to refer to two or more clusters of ivy, as you initially asked, you can use ivy plants.