Sycerika McMahon has missed out on the Olympics 'A' standard for the 100 metres breaststroke by 0.3 of a second at the Celtic Tri-Nations meeting.
The 17-year-old Leander club swimmer from Portaferry set an Irish record of 1:08.79 in Friday's final in Dublin.
McMahon will have another chance to make the qualification standard at the European Championships in May.
Ireland also won the men's 200m butterfly, 50m backstroke and women's 4x200m freestyle to lead after day one.
The event featuring the hosts, Scotland and Wales, continues on Saturday and Sunday.