Yeah, go for it Katy -- you'll be fine.
As for the weather, it will probably be all of those things and more, knowing the west of Ireland. Everyone's different, but personally, for the race itself I would go for the teeshirt or vest option, regardless of the weather.
Can't wait for the report. We may need to pick your brains about the race as a few of us have expressed an interest in doing it, or the half, next year.
As for "I am never, never, never going to do this again" --
-- we all say that, until the next time.
Enjoy the Guinness rehydration.