Hi there. I assume your main concern is Lough Ree if you are coming from the Grand? My suggestion is to maybe spend the night at Coosan Point or Hodson Bay and then go for it across the Lough at first light, when the lake is generally calmer. Put your nose out and see what it is like - if in any doubt, turn back and try again tomorrow!
Take any advice from the lock keeper at Athlone, as they will know the forecast well, and the lake is affected differently by different winds. It is always a good idea to go across with another boat - safety in numbers! And definately take a good pair of binocs as the markers can be very hard to spot!
If you are in a canal boat it will probably take 4 - 5 hours to cross in good weather.
In my experience mobile phone cover is pretty good on the lake - none of the holiday hire boats have any other type of radio except their own mobiles! If you are really worried though, maybe try and borrow a VHF.
Once you have got as far as Quaker Island you have passed the worst (three quarters across), but unfortunately there are no moorings to pull into from Coosan or Hodson untill this point, and if you need to get somewhere here then try for Lecarrow. There is also Barley Harbour but it is not as sheltered.
Once you can see the smoke from the power station in Lanesboro you are pretty safe! The rest of your journey up the Shannon should be plain sailing. One of my favourite moorings is at Drumsna - quiet, sheltered and walking distance to pub and shop.
Have a great trip!