I've done the Grand Canal twice, from Portobello to Shannon Harbour. The last time was last Novemeber, but I wouldn't recommend doing it that late in the year, the ground is too soft and it gets dark too early. It took almost two hours longer by going later in the year. There's not too much prep needed, but a few things to consider,
- The canal spits at Robertstown, you want to go right. Left is for the Barrow line.
- Roberstown is also one of the few places to stop for supplies.
- Decide what average speed you need to attain in order to make in on time and try to stick to it.
- No need for a full suss, hardtail is fine.
- There's some amazing scenery, but there's not much time to enjoy it, have to keep moving.
Here's the Strava from the last time,
https://www.strava.com/activities/765817399