Ryanair's policy on anything firearms or related is simply NO. That being said and utterly hypothetically of course, as no one would advise or condone lawbreaking. If it was to be put in the hold luggage at Ryanair's exorbitant baggage prices in an unassembled state amongst much clothing or whatever...However,as you have posted on a public forum as to where from and where to.I wouldn't advise it.

I thought there was no max in Irish law when it came to restricted rimfire ?

Makes sense if you plan on ever buying the 10 round mag

Not a good idea. An Post prohibit any firearm component in inbound international mail. As mail from the US now has to have full customs declarations before it is even processed here, there is a very high chance your package would not get through. Quite possibly end up just being returned to the US....., eventually.

Same risk. Up until a couple of years ago, postal customs was done visually with little or no reliance on any electronic declarations. Now it's the exact opposite.

You were lucky. Some people still will be but a lot won't.

The original OP was talking about bringing them via baggage on a Ryanair flight, whether by hold baggage or hand luggage[a VERY bad idea],not by mailing them from the UK.which can be done and you don't mind finding the correct customs declaration [TARIQ] number and paying duty on it from the UK.

 I have checked An Post's policy on prohibited items each time before ordering and see no mention of magazines.

True,because you are not shipping the items by AnPost.AP is carrying the items that were cleared by Royal Mail.So if RM has no problem shipping magazines or other components,then AP or any other mail carrier will do likewise.Where it becomes sticky is when AP delivers this to customs and Excise in Ireland and they decide whether to release the parcel with a customs fee,or hold it as its suss to them.