Generally, nowhere's too far from the course.
Of those I've stayed at..
Douglas Rugby Club, on the course, staggering distance from the Quarterbridge pub, not sure what they charge now but think we paid £10/night/person. Toilet/shower faciilities weren't exactly brill. I'd guess they've improved by now?
Glenlough, again on the course, a good 40min-1hr walk to Douglas, bus stop outside though. Better facilities wise, onsite beer tent/bar. Can be populated by the 'enthusiastic' types who like to rev an engine to pieces.
Union Mills footie club, best shower/toilet facilities I've experienced (in fact one year while staying elsewhere we'd nip there just to get a decent shower!). A ten min walk to the course and the Railway pub, up hill on the way back can be fun when on your 'beer scooter'! Clubhouse servers grub, beer, has TVs too. Bus stop there too.
Claddaghs, fair way from civilisation relative to the others, but FREE. You simply need to apply for a permit. Lovely setting next to a stream. Toilets and wash basins but no showers (my lad decided to jump in the stream..it was freezing so he didn't do it twice!). Walk 0.6m one way and you're at the Sulby Hotel, great to watch for sheer speed, same distance the other way and you're at Ginger Hall, nice famous place to watch from with a pint in your hand (but take a torch as the walk back is along a blummin dark lane). Bus stops at Sulby and Ginger.
As I say, my info is a couple of years out of date, no doubt others will have more recent, and quite possibly different experiences.