The only thing posting it to yourself does is provide some manner of proof of prior creation. This doesn't help always with patents (see
wikipedia blurb). It also doesn't really help with copyright, except that the fact that you independently created it before a certain date may be useful to defend infringement proceedings brought against you (as opposed to being helpful for you to bring proceedings against someone else). It may be useful if someone rips off your work but it's only persuasive, not dispositive.
It's certainly no harm to post it to yourself and keep the sealed envelope somewhere safe.
Unless you have filed a patent, you have no protection for your idea (outside of trade secret protection) so could only sue someone if they deliberately stole YOUR idea (rather than them coming up with it independently themselves).
Also note that you do NOT have a year from first disclosure to file a patent application in the EU - the only permitted disclosures are at certain approved exhibitions, which are unlikely to apply to your case.
If I was you, I'd do a consultation with a patent attorney (if your patent would be applicable in the US, I'd go to a US attorney rather than an Irish patent agent) and have a chat with them. They'll often do it for free or little money as an initial consult, banking on you going ahead with the patent - you'd have to bear in mind that they would have a vested interest in your applying for a patent, but you will definitely get some good information.
If you talk to DIT, make sure you have a signed NDA before you disclose the patent. I'd also contact Enterprise Ireland for advice (it may be useful) - ditto for the NDA if you go to EI.
If you are prepared to do a lot of the work in drafting the patent application yourself, you can cut down the costs of patent attorneys significantly. But, as you and others have mentioned, it's costly to even sue someone for infringement, even if you have a patent...
