This is another subject flagged by the ambiguous wording of Part FCL.
First of all, a bit of history.
A full IR course from scratch consists (if you do it all in an aircraft) of 50 hours SEP and 55 MEP. The theory is the same as ATPL so, for instance, if you have a VFR CPL as I have (or did) you will need to redo many of the ATPL exams again.
The UK has the IMC rating, which goes on a part FCL licence as an IR(r). This allows you to fly and do approaches in IMC but gives you no extra airspace privileges so you can’t fly UK airways or any other class A airspace.
Many UK pilots have an IR(r) and have amassed many hours under IFR. Others, given the airspace restrictions, do little else than revalidate/renew.
EASA introduced an IR based on pilot competence (Competency Based IR) which goes on your licence as a full IR but is a shorter course based upon the pilot’s ability which is assessed in a flight with an instructor prior to commencing the course. There is a reduced theory course which mandates two days ground school instead of three weeks. The exams are simpler and directed towards piston engine flying. ATPL exams, of course, count. If you have CBIR theory only and an IR you cannot fly jets until you have done a bridging theory course.
The course is to be taken at an ATO. Dual IFR instruction and prior IFR flight can be taken into account. Let’s now split it up into Single and Multi-engine courses.
Both courses are ten hours shorter. Hurrah!
Training consists of 40 hours dual but prior training and P1 flight with an IR(r) can count towards this up to 30 hours.
Having said that there is a minimum of 25 hours dual instrument instruction (10 in an ATO) that must be flown.
Wow, that couldn’t be clearer, could it? So here are a couple of examples.
Example 1 Train for your IR(r), 15 hours (25 left). Fly P1 under IFR for 15 hours (10 left). Fly 10 hours at an ATO, Bingo!
You have the 40, the 25 dual and 10 ATO hours have been met and you have a full IR
Example 2 Train for your IR(r) 15 hours. Train dual with an IRI outside an ATO 10 hours, Train 15 hours at the ATO
Either way you need 10 ATO, 25 Dual and 40 in total. A bit easier to digest!
This is where the fun starts.
Training is again 10 hours shorter so consists of 45 hours dual instrument instruction but prior training and P1 under IFR can count up to 35 hours.
Again, a minimum of 25 hours dual must be flown with the addition of 15 dual in a multi of which 10 must be at an ATO.
Example 1 Train for your IR(r) 15 hours (30 left). Fly P1 under IR for 15 hours (15 left). Fly 15 hours in a multi at an ATO.
Example 2 IR(r) 15 hours, P1 IFR 10 hours, Dual outside an ATO - SE 5 hours, dual outside ATO - Multi 5 hours, Dual at ATO 10 hours. More complicated but potentially cheaper of you can find an instrument instructor and multi aircraft outside an ATO.
In summary, for a MEP IR you need 10 ATO, 25 Dual and 45 total of which 15 needs to be multi instruction.
Of course, none of this includes pre-entry assessment flight or skill tests.
The rules appear ambiguous and many people have failed to interpret them. I agree they could be worded better but essentially, if you don’t do any training in a simulator, the above hopefully makes it easier to digest.