the post office should have given you a white certificate of posting which has a number on it, go to your post office and get a form P58 make a claim if it is up to £46 in value that may have gone,
If you took the item to a post office to be weighed and posted from there then the staff will have ensured that you paid the correct postage .
If , however , you just put a 2nd class stamp on it then that will not be enough to get it to Ireland .
In the latter case the item will be returned to you if you have put a return address on it .
The item will also be returned to you if you have got the addressee's address wrong or if there was some other reason why it couldn't be delivered .
If you have not put a return address visible on the outside of the package then the package will need to be officially opened to see if there is a return address on the inside .This would be done by the returned letter branch in Belfast (if the item was being returned to you by the Royal Mail ) but I'm not sure what the procedure would be if it was being returned to you by the Irish Post .
In any case I think you would have to wait 28 days before it was classed as missing .
I work in a Post Office. All you need to do is pop back into your local Post Office and ask them for a claims form for lost post. Then fill out the form, reading all instructions carefully. Send off the form.
unfortunately this happens a lot with eBay
Always send recorded delivery, no matter what the item or the price because some buyers do it for a 99p item - just add the recorded price to the P&P on the listing
As it's eBay, I would refund the buyer voluntarily as if they make an INR claim and you don't refund yourself but make it go all the way, then it sits as a mark against your account. Too many marks and you'll find your a/c sanctioned.