The UK site shows that the original UK price was £34.99 (and then subsequently reduced to £26.24). The lamp is now being offered at half the original UK price. That's probably because the item has been slow to sell in the UK, so Argos want to clear their stocks, whereas sales in the Irish Republic have been strong enough to justify retaining the original price.
It's common practice for large store groups to charge different prices in different areas, depending upon local conditions. They should be congratulated for their sound commercial practices, not criticised for them.
Chris