IRR is defined as the rate of return that sets your portfolio's net present value to zero and represents the annual growth that your portfolio is expected to generate. For portfolios with an effective duration of less than 1 year, IRR can be particularly volatile and should not be considered absolutely reliable.
As reported in your portfolio dashboard, IRR is calculated using the cash flows that are comprised of values (net of fees) and dates of contribution events, distribution events, and the current unrealized value of all of your investments (which includes both mark ups and mark downs). Since time is taken into account, your portfolio's IRR changes over time, even if you haven't made any new investments.