Amongst many other things, these batteries are not allowed:
Batteries that are classified as dangerous goods and certain used batteries (including wet spillable lead acid/lead alkaline batteries (such as car batteries), used alkaline metal, nickel metal hybride (NiMH), nickel cadmium (NiCd), zinc-air batteries, and damaged batteries of any type).
Batteries, specifically new and used lithium batteries when not sent with or connected to an electronic device.
The person at aardvark5's post office is unaware of their restrictions.
I asked a uk seller of ebike that would be good size for wife it’s a small frame would it ship to Northern Ireland from England came back said no as of lithium battery shipping policy I did watch a utube video of a ebike with exploding battery and now I can see there issue
for a courier to take your battery, the courier needs the battery to be packed according to UN transport protocol and the sender is insured against any problem caused by the battery to the courier's warehouse and onto the destination.
That means in practice that the battery has to be retrieved by a company with suitable insurance and the package is properly labelled.
We send batteries to NI and other countries in the EU with DPD Europe.