I went on a Biology field trip to the IOM many years ago and we stayed at a school in Castletwon. We arrived at Douglas and went west to Castletown by road and I remember going across this bridge and being told we all had to thank the Faeries which we all did. So, if my memory survives intact some 20+ years, I would say it is on the road betwen these two towns.

It's a small stone bridge by the way.

Good luck ... and send them my love.

Zeb