The Weather.
Very strong winds blow up, predominantly from the SW to W, and can create dangerous high waves with a very short wave frequency in no time at all. They can easily swamp a boat, or cause it to broach if following the boat. ALWAYS check the inland lakes weather forecast for Lough Corrib before venturing onto the lake. The winds are funnelled down the Maam Valley, and what may be a force 3 in Cong can easily be a force 6 around Doon.
Shallows and Rocks
Unless the weather is bright these are very hard to spot. They are sharp, dangerous and unforgiving. Just because it looks like open water - doesnt mean it is. A SAFE SPEED is essential, and a GOOD LOOKOUT is a must.
GPS
GPS is not reliably accurate. The reason that the GPS in your car appears to nearly always be on the road is simply because it is programmed to "stick to road" - even when the information it receives indicates it is not.
In areas of the lake surrounded by trees or hills the GPS can be wildly inaccurate, and at certain (variable) times of day a phenomena known as HDOP (Horizontal dilution of precision) can seriously affect the instrument. GPS is an AID to navigation - you must be aware of its shortcomings. It dangerous to rely on GPS as a sole means of navigation.