FPN and nominating another driver

They’ll have to if the nomination isn’t accepted.

Do you really think you can just nominate Mickey Mouse from Florida and not expect to back it up?

Why can't you get the boarding pass or ticket details from your brother?

That still doesn't actually prove anything, though?

I think the relevant provision is Road Traffic Act 2010 s. 35(6), the relevant parts of which read as follows:

Where—

(a) a fixed charge notice is served on the registered owner of a mechanically propelled vehicle . . . and

(b) the registered owner of the vehicle was not driving . . the vehicle at the time of the commission of the alleged offence . . .

the registered owner shall—

(i) . . . give or send . . . a document in the prescribed form . . . stating the name and address of the person who was driving or otherwise using the vehicle . . . and

(ii) give or send . . . such other information within his or her knowledge or procurement as the [Guards] may reasonably request for the purpose of identifying, and establishing the whereabouts of, the person referred to in subparagraph (i).

So, in this case the OP has been requested to provide information for the purpose of establishing the whereabouts of the person said to have been driving the vehicle. The OP must comply with the request unless the information is not within their knowledge (i.e. they don't know it) or not within their procurement (i.e. they can't get it) or the request is unreasonable.

OP should ask the driver for the information the Guards want. Either the driver will produce it, in which case the information can be passed on, or they will refuse, in which case OP can reply to the Guards saying "Sorry, I tried, but I can't get the information you want". Communication with the driver should be in writing (e.g. by email) so that a copy of the exchange can be given to the Guards to show that an attempt to obtain the information was made, and was refused.

You don't haved to prove that someone else was driving your car. You just have to tell the Guards that someone else was driving, and then assist as required in identifying/locating that person.

If the Guards suspect that, despite your protestations, it was actually you driving the car, it's up to them to prove it. But if your story that it was some other person turns out to be obviously bogus, that may form part of a case that it was actually you; why else would you have lied about it?

If you send off a notice saying, e.g., "my brother Joe, who lives in Florida, was driving" and that information is, to your knowledge, false or misleading, that's an offence under Road Traffic Act 2010 s. 40(6) for which the maximum penalty is a fine of €2,000. If you own up immediately when challenged, that's probably the only charge you're looking at. But it sometimes happens that, when that first lie is questioned, people double down and tell further lies, getting deeper and deeper into a hole, e.g. by involving someone else to back up the initial lie. At that point you are looking at a charge of perverting the course of justice and, depending on the circumstances and on how big a mess you've got yourself into, possibly other charges.

(There's an ex-judge in Australia whose reluctance to accept and pay a $77 ticket for doing 60 km/h in a 50 km/h zone ended up costing him a three-year prison sentence, disbarment, public humiliation, and the complete destruction of his career and reputation.)