What’s the climbing like ? I’d like a go if I can fit it in but would need to think about choice of gear.
Plenty of it. If you read Pete M's A-Z account he talks of the tarmac being glued onto the side of a cliff or some such. First 620km is fairly tough, get through that and you are on your way. Next 1000km the terrain eases though it's still hilly and of course you have that Atlantic wind blowing away. Then as you enter Sligo, Lietrim, then Donegal the steeper stuff returns.
Expect both Alpine 5-10% type grades and 25% grades along the route. It is a stunning route and looking both forward and back and realising you have ridden or will ride every headland (that has a road) in view and beyond is quite a feeling.
Plus I believe Eamon has found us an extra hill on Achill Island to keep the kegs warmed up.
Ivan did the Transatlantic Way race on fixed, which is basically the opposite way north to south. He may have an opinion on gear choice.