This afternoon I've installed a gateway up on Onchan Head. It doesn't quite have the line of sight of the whole of the bay I was hoping it would have, but I'll do some mapping over the next week or so and see what the coverage is like. It is a Things Network gateway with an external antenna (quite a broad frequency range), connected to a portable 4G WiFi device.
My initial plan today was to install it at Carnane, but unfortunately the new building I have access to is on the wrong side of the hill, so no use for Douglas.
If it all behaves itself over the next few weeks, I'll look at getting it up Snaefell and installing it with the antenna facing south and see what the coverage is like from up there.