Mountain biking in Ayrshire mostly happens on the northern edge of Galloway Forest at Carrick Forest near Straiton. The trails are waymarked through forestry, technical enough to be interesting without being a black-diamond commitment, and ridable year-round if you don't mind mud.
The picks below score MTB-tagged activities against wind and visibility — rain is less of a deal-breaker for MTB than for road cycling, so the scoring weights it more lightly.
Straiton, South Ayrshire
Waymarked forest trails on the northern edge of Galloway Forest Park.