I’ve not lived on the Island for a few years, but the enduro scene appears to be booming and there is a huge amount of trail building going on.
Snaefell is a long push up and the footpath will be busy during TT. I would also avoid cycling on the mountain road during TT.
South Barrule is a far better option for a summit, and can be linked into a good route far easier (Cringle, Arrasey, Glen Rushen, Kerrodhoo and South Barrule plantations all close by).
Union Mills is at the foot of the hills so to get to most of the good riding you’ll be looking at a 30+ mile loop. If you want something like that I’ll have a dig through my strava and see what I can come up with.
The ‘Peaks Challenge’ course in Windysurfer’s link is good and starts near Union Mills – horrible green lane climb into the hills then mostly singletrack footpath back down.
Chibbannagh and Archallaghan plantations are both easily to reach from Union Mills and would be good on the SS but far from the best riding on the Island.
Up Peel hill is a nice run on a sunny evening
One of my favourites – stop in Peel for ice cream/beer/fish and chips after, nice cruise along the old railway line back to Union Mills