Definitely need to get the ferry and accommodation booked, as most folks that come over are repeat visitors who book their trip 12 months prior.
If you struggle to get a hotel, B&B or homestay accommodation; then bring over a tent and camp. A few of the good ones are Glenlough, Glen Wyllin and Peel.
The 1st week is practice week which means that the roads are open all day and then they close in the evening for a few hours practice. You won't get to see the actual races but it will be quieter and give you more chance to ride around.
The 2nd week is race week which means that the course will be closed for the races on the Monday, Wednesday and Friday. (Unless they need to use a provisional day due to bad weather) You will obviously get to see the races though and the Island is busier with a better atmosphere.
When you're out and about on your bike make sure you ride within your own limits and don't get encouraged to ride quicker than you're happy with. No matter how fast you are on a bike, there is always someone faster or someone prepared to take a risk. The roads are really great to ride on but do not give any room for error. There are plenty of stone walls, hidden junctions, tracks and the rest of the population are out and about doing normal day to day stuff.
It's definitely a great event to come and see and the numbers have increased over the last few years; no doubt partly due to the excellent coverage on ITV4.