Ulster, Leinster, Munster and Connacht are all in action.
After all the disruption due to Covid-19, five players will make their Munster debuts in Sunday's Champions Cup opener against Wasps in Coventry.
Academy quartet Patrick Campbell, Scott Buckley, Eoin O'Connor and Daniel Okeke have got the nod, as has prop James French.
Coach Johann van Graan is also able to call upon Ireland internationals Peter O'Mahony, Conor Murray, Joey Carbery, Andrew Conway, Keith Earls and Tadhg Beirne for the encounter.
Leinster play Bath at the Aviva Stadium tomorrow and the province has revealed three players are isolating after testing positive for Covid-19.
Head coach Leo Cullen has named a team including Irish players Hugo Keenan, James Lowe, Garry Ringrose, Jamison Gibson Park, Andrew Porter, Tadhg Furlong, Ronan Kelleher, Josh van der Flier and Caelan Doris.
Ross Byrne will play at out half and Rhys Ruddock will skipper the side in the absence of Johnny Sexton.
Ulster also play tomorrow, away to Clermont Auvergne in France.
South African World Cup winner Duane Vermeulen has been named in the back row for the province.
And Connacht host Stade Francais at the Sportsground on Sunday.
Jack Carty will captain the side with Shayne Bolton starting at outside centre.
There's also a new look second row of Oisin Dowling and Niall Murray.
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