Pro14 champions Leinster say they are assisting two senior players who have tested positive for Covid-19.
Leinster confirmed three days ago that all players and staff tested negative for Covid-19 after the Boxing Day derby with Munster was postponed.
It was called off as Leinster were "investigating the outcomes of some of the results" from Covid-19 testing.
"A number of close contacts of the two players have been identified and are also self-isolating," added Leinster.
"One player was tested outside the PCR testing window, after feeling unwell, and is now self-isolating.
"Further PCR testing was carried out at Leinster Rugby yesterday (Monday) morning on 104 players and staff and 103 tests returned negative results with one senior player testing positive and he is also self-isolating.
"The HSE has been informed in accordance with public health guidelines. The Leinster Rugby squad return to training later on today (Tuesday)."
Leinster are back in action in action on Saturday with a Pro14 encounter against Irish rivals Connacht in Dublin.