You raise two very important issues here Deano.
In the case of Rangers and Celtic fans, Inverness is no different from other communities outwith Glasgow where they also proliferate. In Inverness, I believe there are two main reasons for this. Firstly, and this is a factor which applies across Scotland, Glasgow's toxic religious and political mix has, over many years, created two opposing sectors in society down there and these two football clubs have become conspicuous features of that divide. This has given these clubs sufficient critical mass for them to act as footballing black holes which drag in additional support from areas a long way away from Glasgow. The whole of Scottish football suffers from this.
The second issue in Inverness is that until just a decade or so ago, the place didn't possess a major force in football. As a result, a lot of people could have their "big" team - usually the OF but possibly Aberdeen etc as well - and also their "wee" team, in other words lower league ICT or before that Jags or Caley. These could comfortably coexist. However now that ICT is also a "big" team some fans have had to make a choice and some of these have remained with their OF preferences.
Hundreds of Invernessians regularly disappear down the road to Celtic Park and Ibrox, but I do believe that, as time has passed and ICT have become more successful, this unfortunate trend has already begun to dwindle. Evidence is largely anecdotal, but I think OF support in Inverness (especially Rangers for obvious reasons) is on the wane.
Regarding the second issue of lower home gates now than several years ago, this is largely part of something that is happening across Scottish football, it's not just ICT. Football attendances do go in cycles and there are probably several reasons for the current downturn. I would include silly kick off times, unavoidably high prices, lack of personal funds after the recession began and competition from a range of non-football activities.
So although ICT's success has provided motivation for more to attend, it has been difficult even to stand still against these other negative factors.fair enough