This confuses a lot of people.
In order for an IR to be valid on multi engine aircraft, it needs to be completed in a multi engine aircraft.
Single engine IR privileges are included in a multi engine IR, but not the other way round.
You could do a single engine IR and then upgrade it later to a multi engine IR, but there is absolutely no benefit, it costs pretty much the same.
So you need to get a single pilot aircraft multi engine instrument rating (SPA-ME-IR) in order to apply for airline jobs.
When you then get an airline job, you do a type rating which is effectively a whole new instrument rating, although this time it's multi pilot multi engine, so you end up with MPA-ME-IR.
These are two separate ratings, both would include single engine privileges, but clearly you can only fly single or multi pilot depending on what you keep current. If you want to be able to operate both single and multi pilot IFR, you need to keep both ratings current.