You can't use the IRR function because your cash inflows do not end -
this is called a perpetuity.
IRR is the rate of return (or discount rate) at which the net present
value (NPV) of a set of cash flows is zero. The NPV of a perpetuity of
say £150 per annum at a discount rate of 8% is £150/0.08= £1,875.
In your example, there is an initial cash outflow at the beginning so
the NPV is the same i.e. If the outflow is £2,000 the NPV is -£2,000
(using the convention that outflows are negative and inflows are
positive).
So, using the figures above, you need to find the discount rate i
where:
NPV = 0 = (150/i) - 2000
2,000 = 150/i or i=150/2000 = 7.5%
This assumes annual cash flows.