I think in our current times we would say that they are the same. See other answers. Really the answer is "yes, but no".
We need to think historically. Where does the Ionaian mode come from? Early music? Gregorian chant? "Modal" music? The Ionian mode is an outgrowth of the Lydian mode for voice leading purposes. In F-Lydian there is a B-natural and in F-Ionian there is a Bb. It was common practice in early church and secular music to "raise" the fourth tone (B-natural) when ascending and "lower" when descending. But it is important to note that Lydian was actually more commonly used.
It's also incredibly important to note that the idea of Ionian and its relation to the Greek scales is completely made up by Mediaeval Europeans. We don't know what the Greek Ionian scale is, there are only loosely academic ideas, but nothing definitive.
So, back to Gregorian chant. The "modes" were actually just constructions and definitions of common scales used in songs that people were singing. Theory almost always follows practice. Rather than the 7 modes we think of now, there were 14. Ionian; hyper-Ionian. Dorian; Hyper-dorian. Etc, etc... These names were used because they defined the range and common cadences of each set of pitches. Ionian and hyper-ionian were indeed what we would call a major scale, but they were used in functionally very different ways.
First and foremost, to say define common chord movements in Ionian vs C-Major is denying the fact that Ionian stems from "pure" counterpoint and voice leading. Yes, in modern contexts we thing of modal harmony. However, If you dive into the works of say Palestrina, you will see how different he uses the Ionian and how different it is from a Major Scale. The major scale indicated tonality, and often the shifting of tonality, more in line with our modern ear.
You can look in Palestrina's music and see that while he only uses "the white keys" (with exceptions for voice leading), he cycles through many, many modes. He has work in E Phrygian, which shares all the notes of C Major, later he will move into A Aeolian, which shares all the same notes as C Major, and then into C Ionian. Of course A Aeolian is different than C-Major even though they share the same notes, but so is C Ionian. It's about the specific ranges, movement, and function of each scale. This is a very different musical world to our current one.
So history is important, but even more important is historiography. Modes are now used in many. many different contexts and this is because our relation to these historical constructs are constantly changing based on how they are interpreted. Right now, as you can see from the answers, there may not be a huge difference between C Major and C Ionian. However, historically the difference is substantial.