"A.R.T." wrote:
How can you calculate the rate of return for a
series of payments not necessarily equal?

There is a recurring misconception in the responses
to such questions.

Use IRR whenever the payment __periods__ are equal,
even if the payment amounts (aka cash flows) are
unequal.

Use XIRR whenever the payment __periods__ are
unequal, whether or not the payment amounts are
equal.

Your question is ambiguous with respect to which
case applies.

Note: If the payment periods are "equal", e.g. "once
a month", but interest is compounded daily, you might
want to use XIRR since each month contains a different
number of days (compounding periods).