Stevenage centre-back Iro, 27, and Brighton striker Barker, 21, will stay on loan at Underhill until 22 January.
Holwijn is a 32-year-old Dutch winger who has signed on non-contract terms.
"We're delighted to have got Andy in on loan to further strengthen our defensive line," said Bees joint head-coach Mark Robson to the club's official website.
"George potentially has a lot to offer us. We hope he'll get the opportunity to show his qualities and to give us another option up front."