Sometimes the fancy spreadsheet is the problem! The reason I say that is without a good understanding of what it's doing, it's easy to create your inputs in a way that wasn't what the spreadsheet "expects".
To calculate internal rate of return, you'd need the starting balance, all deposits and withdrawals that would affect the balance, plus the date of each of those. And the last bit of information would be the balance now. I've posted a simple spreadsheet where the portfolio starts with 10,000, $250/month is added, and there are random gains each month recorded. The IRR would have been 4.1% without the $250/month, but is 3.7% with those monthly deposits.
https://drive.google.com/file/d/0B6X74x23Hx7DUXRObkhxZ0duOEU/view?usp=sharing