First off, your IRN's dont need shell grit at all.
Grit, in the purpose of birds, is for aiding the digestion of whole seeds (ie; a pigeon or dove that doesnt 'hull' the seed itself, it eats the thing husk and all, the grit aides in taking the harder outer off.... psittacine (parrots) do not need it as they take the outer off before ingesting)
Regarding calcium, there are actually three main things that take place in regards to calcium and diet...
1) calcium
2) phosphorus
3) vitamin D3
All are heavily interlinked.
Without going into a huge amount of detail (i will provide a link for a more indepth look at it) you can actually ditch the cuttlebone / mineral block all together.
My pet birds dont touch cuttlebone (just like im sure an extremely small percent of wild parrots actually do either)... and it might be worth noting, that a bird who really does like its cuttlebone and has it given in large amounts, can actually do quite alot of harm in the long run (too much calcium is actually very bad)...
Food will give your bird more than enough calcium, and at a proper ballance as well. Fullfilling many goals in one hit... (diet needs, food, treats)
You can (on a very basic diifficulty level, and with alot of joy for the bird) solve calcium, phosphorus, and d3 in these tiny steps...
1 cup of Broccoli or Spinach a week (calcium)
1/4 cup of peanuts a week (phosphorus)
1 boiled egg a fortnight as well as some proper daylight (not windowsil, its useless as it filters the UV rays to points of nothingness for the bird) or a proper Uv light on / near the bird cage. (vitamin d3)
Doing something like that will cover more than enough (supplemented by other fruits / veg throughout the week/fortnight) to give your IRN a healthy happy life.
helpful info:
http://www.birdsnways.com/wisdom/ww22eii.htm