How hard is it for a prior-service Marine to transition from Army IRR to active duty Army?

The Reserves are support MOS's. They don't have combat units (for the most part). They are things like admin units, finance, medical support units, communications, or MI units. They augment other deploying units. The National Guard is combat units that are mostly set up like the Active Duty Brigades Combat Teams. This is where you'll find your infantry and cav units. NG units vary drastically from one state to another, they each have their own personality. One NG state might be great to work with, while another might be terrible. For instance, Texas NG was awesome every time I worked with them.
As for being released from the Reserve Component to go active, that is at the discretion of your command team to approve or disapprove. Experiences vary. Guard units deploy about once every four years or so. You can also do an interstate transfer to deploy with a different unit, but again, experiences vary on whether or not it will actually get approved, it's based on unit needs and not your desires.

Sgt Landon W. to clarify, the Combat arms units (armor, infantry, artillery, etc) are in the ARNG. You will find some engineer units in both the USAR and the ARNG. So if you want Armor or Infantry etc, then the ARNG is the Route. the USAR Engineer units are part of Theater Engineer Commands with a variety of Engineer functions, not just combat engineers.

Being prior service, it is usually your former MOS that gets you back on AD. My recommendation is to go talk with a recruiter from each component (AD, USAR, and ARNG) and see what your options are as it is a moving target. USAR and ARNG recruiting are against current vacancies so you won't get snowed about opportunities. Not saying AD Recruiters will snow you, but they are filling slots in a much bigger organization with more diverse needs. Today's priority may change.

As for your friend who said <insert comment here about the guard>, put on your PM hat here: would you procure a new IT system or piece of software on one review? No, of course not. You'd use a deliberate decision Support tool based on data and verified information.

So you left the reserves, what is different now that make this viable? As for ARNG deployments, they are generally on a deployment + 5 years cycle. I say generally. It is really what force capabilities the Combatant Commanders are requesting and how the services elect to fill them. You can't generally wargame it off a deployment. There's is also the Tour of Duty MOBCOP that you can jump on a deployment as an individual.

So, hi.
Prior Service AD Marine switched to AD Army. The process was pretty easy. As far as jobs that’s up to the in and out call I went infantry to infantry which is under strength by a lot so it should be easy.
Hardest part will be finding a recruiter to give you the time of day. Give me a shout if you want i might be able to point you in the right direction