Gubu.ie

How best to sell one sovereign 1911 and three half sovereigns from 1990's...I would say they are 'mint'.

I've used eBay quite successfully for the sale of sovereigns but with all the messing with Brexit now I have given up using them. An auction by one of the specialist salerooms such as Weldon's http://jwa.ie/ or O'Reilly's https://www.oreillysfineart.com/ both Dublin based. Full listing of salerooms here: www.collectireland.com You should get them accurately weighed before doing anything and check that the actual years are not more valuable than the weight of the gold in each coin.

Would weight be significant issue if 'mint'? seems expensive route to go with auction house and is it 20% commission. Too valuable for ebay? Must make enquiries... and I see online auctions nowadays, could be addictive!

Not too valuable for eBay and it may even still be the best place. Certainly you may be able to check prices there for your specific coins. Auctioneer's fees frequently exceed 20% but this must be weighed against the worldwide online exposure that most auctions now achieve.
I forgot to mention in my earlier post that there are also some useful sites here: https://collectireland.com/numismatics/

So with an Ebay sale, would you register the item when posting and how would you receive payment? wait for it to arrive? seems bit open to fraud??

On eBay you set a reserve and are paid up front and in the case of the coin you would register it. I would always disguise it too - I have usually used a hollowed out book in the past. However, if you are new to eBay you may well find it difficult to sell there having no feedback. Also, fees are incurred when selling on eBay. Realistically, you need to get your coins valued and then get them included in an auction in Ireland that lists on the various British online platforms - https://www.easyliveauction.com/ etc.

Why mess about with ebay. I'm sure you could sell it in Ireland with out all the hassle of fees postage ect,ect
Just do your due diligence on the pricing, and advertise it. Auctions are going to command commission and you wont feel it until your bit of profit has been wiped out. If its only 4 coins there should be no problem.