I remember the IMC mostly for way they used to kindly make their online reports available to us the night before the official press launch. It was born in controversy after the IRA once more came up short for David Trimble in October 2003. At first, it was attacked by both Sinn Fein (its primary object of scrutiny in the early days at least) and the DUP.
Much of the controversy revolved around the fact that in compiling its reports, it used intelligence reports from the PSNI, Garda Siochana and the various military intelligence agencies on both sides of the border. However as time went on (and presumably compliance levels increased) it’s reports began to be used as proof of improving behaviour.