Or if you can get a better deal on 3, count me in too.
In relation to the darts, August the 9th is a Sunday. The first dart leaves Bray at 9am ish and the first dart from Dublin leaves at 9am ish too. So the darts wont get to the Merrion road crossing till 9.10am at the earliest anyway.
Given that typically the IM races start at 7am and and you have only 1hr 10mins (before being DQ'd) to complete the swim you should have well passed the Merrion road crossing before that time. It is only about 7km away.
I also see that they are setting people off in waves every 10mins over a 1hr 15mins period. If you are in a later (last) wave starting at c 8.15am and it takes you 1hr 10mins to complete the swim, you should be at the level crossing by 10am even for the slowest swimmers and cyclists. I am sure they have something worked out either way whether no darts for the first hour or no darts at all for maintenance. Anyway I think if you haven't passed the crossing by 10am, the dart will be the least of your worry/frustration.