Based on what co workers in Ireland (Citywest area of Dublin) say....you will get rained on. so fenders of some sort are probably a requirement.

Your budget would help

First thought is road bike, with drop bars, not a mountain bike

Do you have any ideas of doing triathalons (you noted ultra distance swimming...might be a natural progression)

Quick options:

Touring bike (already mentioned)

Road bike, good quality components but not full race....various ways to fit fenders or smaller fenders (sks race blades)

Cyclocross bike (aready noted)

Bike clothes, clipless pedals, and good fit will make your life better.

go for it