I attended a webinar on this topic recently and the key takeaways were:
1) check where the seller is located - .ie or .eu web domain is not enough
2) check where the goods are made - if made in Britain then no customs duty should apply (examples given were hotpoint, dr martins, marmite, cadbury, certain cars)
3) check who is liable for these extra charges - some sites are dealing with it on behalf of customers so that product arrives DDP (delivered duty paid), others are not and as the buyer you become the importer for customs declaration purposes
Calculations also make things complicated.
For VAT purposes the value is the (cost plus handling, transport and insurance, plus any customs duty). Its this "value" that needs to be under the €22 limit, not the cost of the goods only. Irish VAT is calculated on the value. (No UK VAT should be charged)
For customs, the value is the cost plus handling, transport and insurance to see if the €150 threshold is met. If value > €150, then customs duty is applied to the cost only.
The real fun starts if you need to return goods to the seller, complete export declarations and try to reclaim the related VAT or customs duty paid!