The Intel Rapid Storage Technology (IRST) driver presumably enhances the performance of a HDD/SSD. Many users find no difference in the performance of their drive(s) whether IRST is installed or not. Others find a slight improvement. In any event, IRST has NOTHING to do with disk capacity, so it's not clear why you're mentioning it in the context of your query.
I believe the actual capacity of your 64 GB SSD is somewhere around 60 GB. Apparently you've created a 19 GB partition (I suppose it contains your OS). The remaining unallocated disk space is 41 GB, so it appears that the system is reporting the full capacity of your 64 GB SSD. Naturally you can create a partition out of that unallocated disk space and use it for any purpose you desire.
Please understand disk space capacity has nothing to do with a drive's "cache". You might want to Google "disk cache" for a thorough explanation of what that is.