In simple terms, credit is available towards the CbM IR for pilots with previous instrument flight time. Simply logging 'IFR' time is pointless; the CbM IR requires 40 hrs of instrument flight time, at least 25 of which must be 'dual flight time' - and of which 15 may be under instruction for the IR(R).
So that leaves another 15 hrs of instrument flight time, which can be further dual training or as PIC of aeroplanes with an IR(R).
In summary, you could in theory claim 15 hrs of dual instrument flight time plus 15 hrs instrument flight time as PIC of aeroplanes towards the CbM IR, then just fly another 10 hrs of instrument flight training before the skill test.
The credit was poorly explained in the past - it said that credit was available for 'prior flight time under IFR as PIC on aeroplanes, under a rating providing the privileges to fly under IFR and in IMC', which was further qualified in AMC/GM to state that such credit must not exceed instrument flight time.... That has now been tidied up in Regulation (EU) 445/2015 to read 'prior experience of instrument flight time as PIC on aeroplanes, under a rating providing the privileges to fly under IFR and in IMC’.
Instrument flight time is defined as 'the time during which a pilot is controlling an aircraft in flight solely by reference to instruments'. So 'IFR' time in gin clear VMC won't count, it has to be 'real' IF!