Irene is Mach 8 so she's easily the faster. Especially since Minato is only hypersonic through a pretty contrived powerscaling and rumors. On the other hand if he can use summons those may turn the battle in Minato's favor. Otherwise he's getting sliced to pieces the moment he enters Irene's range.
@MrASSH0LE: Based on her quicksword speed which can generate more than 40 cuts on the water in an instant. Mach 8 is only a low estimate really. Mach 40 would be closer to the truth since at mach 8 you would be still seeing a chaotic blur as the path of her attacks.
@Validimus: 1)That would be her quicksword and not her actual movement speed. 2)Speed ranges are measured using the possible distance X can travel in a set period of time.Which leaves skeptic that you could indeed measure that her quick sword is hypersonic unless if you measure over what distance the average sword is swung and mustiplied it by 40.
@MrASSH0LE: It may make you surprised but it was calced exactly in that way. The distance she does with a single swing x40. That's under 0.1 seconds (peak human reaction time). Although as I said it would be more accurate to use the peak human image perception rate which is magnitudes faster. Her sword swings were afterall totally invisible to the most. Also Irene can the very much react at the same speed. Otherwise she couldn't control her sword and couldn't intercept each and every swing of Clare's quicksword. She also more or less kept up with a Teresa who previously blocked her quicksword without any Yoki power. While not as fast as her quicksword, her leg movements are not much behind. Like I said if the Yondamine approaches Irene, he will die.