They are both lovely names.
"Sally" is a good name, and also common and popular. It was originally a diminutive or nickname for "Sarah," but it has become a name in its own right. The first person that comes to mind when I hear the name "Sally" is the late astronaut, Sally Ride. The second is someone in my wife's extended family.
"Ivy" is much less common, and a little old-fashioned. The women's name comes from the plant named "Ivy." Ivy is a nice plant with green leaves. It is able to attach to and climb up brick and stone walls. Old buildings with brick walls often acquire a covering of ivy. It is sometimes planted intentionally because people think it looks nice on a wall. In the United States, a group of seven old universities is called the "Ivy League," because they are old enough for many of their buildings to have become covered with ivy.
The only person I can think of named Ivy was Ivy Baker Priest, 1905-1975. She was the Treasurer of the United States 1953-1961. I know her name because I was interested in money when I was a kid, and a (copy of) her signature was on every US dollar bill.