Hi there
There could be many approaches for counting the RPM but I will suggest you to make use of timers, present in most of the microcontrollers, and count the number of times IR sensor interrupts it in a minute, or you can see it the other way round also, like, measure the time gap between two consecutive interrupts (interrupts from IR sensor) and use them to compute how many would be there in a minute (sort of unitary method).
After you have the RPM, sending message to a mobile number is not that difficult. Here are some links that will surely help you:
1. "engineersgarage.com/embedded/arduino/how-send-message-gsm-module-using-arduino"
2. "engineersgarage.com/contribution/frequency-and-rpm-counter"