Short answer
DSCR measures the relationship between income and debt payments. IRR measures the time-weighted return on cash invested. They answer different questions.
How it works in Koreograph
DSCR helps identify whether income can cover debt service. IRR helps compare when cash goes in, when cash comes out and how large those movements are. A deal can have a good IRR and still be uncomfortable if DSCR is tight.
Developer view
Use DSCR as a finance resilience check and IRR as a capital efficiency check. For UK buy-to-let lending, also remember that lenders may use their own ICR, stress-rate and rent-cover tests.