Leinster have confirmed two senior players have tested positive for Covid-19.
A number of other players have been identified as close contacts and are also self-isolating.
The province's Pro14 game against Munster on St Stephen’s Day was postponed while they investigated some results from a round of PCR testing.
All players and staff tested negative but a further round of tests on Monday morning revealed one positive result.
Another player was tested outside the PCR testing window, which also returned a positive result.
A Leinster statement read: "Leinster Rugby are assisting two senior players who have tested positive for Covid-19.
"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.
"A number of close contacts of the two players have been identified and these players are also self-isolating.
"The HSE has been informed in accordance with public health guidelines.
"The Leinster Rugby squad return to training later on today."
Leinster are due to face Connacht at the RDS on Saturday before hosting Ulster six days later.