If you could only recommend one hike on the entire island, what would it be?

Travelling to Ireland at the start of July and spending roughly 10 days there. It's just me and my wife [m31, f29] and are are trying to plan out which part/s of the country to spend a bulk of our time around.

I, particularly, would love to put a huge focus of our trip on the outdoors, but I don't want to get overly ambitious with planning multiple hikes in different locations for such a short timeframe. If you could recommend one hike that is the hike to rule all other hikes on the island, what would you recommend?

A few pieces of context to consider:
• Looking for a single day hike - no overnighters unfortunately this time around
• No mobility issues; overall quite fit and experienced in front country and back country hiking
• Ideally no more than 15kms +/- and/or 8 hours round trip. Shorter is also very fine.
• No preference on points of interest, though we are particularly fond of waterfalls.
• If more context is required, please ask away!