Well officially 98% of people in Iran are Muslims, with 96% being of the Shia sect...Although this doesn't take into account how many of them have lost their faith. And then what is the boundary for you to call some one a Muslim? As I know a few who don't pray a day but however consider themselves to be Muslim. in the end, in the Quran it says any one who believes there is only one god, and Muhammad is his messenger; then that he is considered to be a Muslim. But then there are others who pray shed times a day etc etc and he too is considered a Muslim.
Now in Iran I would say easy the majority are Muslims, but the numbers are falling specially with the generation that is growing up. I attended a closed door talk in Iran where incidentally this was the topic.. They predicted it ranges between 10-30% for a religious Muslim (depending on the boundary), and about 40% are ordinary muslin, pray 5 times, go to mosque etc etc.. And then 20-30% who consider them selves Muslim but however do not reflect it very well, but ultimately it's the intention that counts.
And then about 10% ish they think consider themselves atheist or who actually physically denounce Islam... But that means they can be Jewish/Christian/devil worshippers etc..
That was inside Iran with a population of around 73 mil ish.. Outside Iran however is a different story.. I think possibly outside, those of Iranian ancestory majority are "non-muslim". Depends on when they migrated away.. Possibly 60-70% are non Muslim.. I can't tell..But don't forget the outside community is very small compared to the population of iran.
Hope that helps