Southampton midfielder Davis captained the visitors, while winger Isgrove was introduced as a second-half substitute for the hosts at Cardiff City Stadium.
Northern Ireland, who this summer will take part at their first ever European Championship, looked to be in line for victory when Craig Cathcart's low shot on the turn found the net on the hour mark.
Wales manager Chris Coleman then introduced Isgrove, who is currently impressing on loan at League 1 club Barnsley and has five under-21 caps to his name, two minutes later.
The home side pushed on and were rewarded in the 89th minute, when Simon Church converted from the penalty spot after being fouled by Gareth McAuley.
Isgrove and Wales now head to Ukraine for a friendly on Monday, while Davis and Northern Ireland will host Slovenia on the same day.