IRR is the most complete return metric you can calculate on a rental property because it accounts for the time value of money across the entire hold period. You enter every cash outflow (down payment, rehab, capex) and every inflow (monthly cash flow, refinance proceeds, final sale) by date, and IRR gives you the single annualized rate that makes those cash flows break even at zero net present value. Most investors never calculate IRR because it is a pain to do manually, but it is the only metric that lets you compare a rental to a stock portfolio or a private investment on equal footing. A rental that cash flows $200 a month for 10 years and sells for a $150,000 gain has a different IRR than one that cash flows $400 a month for 10 years and only gains $80,000 at sale. IRR tells you which actually performed better.
Example
You buy a property for $40,000 down. It cash flows $3,600 per year for 7 years, then you sell for $65,000 net proceeds after debt payoff. IRR works out to approximately 15.2%.