A little bit of googling yourself and you could have answered your own question rather than query advice given.

https://www.visitisleofman.com/isle-of-m...land-guide

From the guide:

5. CROSSING PLACES AND ACCESS CORRIDORS
In any interval, the Clerk of the Course may authorise traffic to cross or proceed
along the Course at the places specified below, at such times as they may
specify:-

 Douglas, St Ninians crossroads
between Ballaquayle Road and
Ballanard Road

etc
etc

plus of course the access road on the old railway for light vehicles under the bridge at Union Mills