Leinster, Munster and Connacht will all name their teams later today for this weekend's European pool games.
Defending Heieneken Champions Cup winners Leinster are away to Premiership side Bath, with Munster hosting the French champions Castres on Sunday.
That meeting is the 15th between the sides, making it the competiton's most common pairing.
Ulster are the first of the Irish sides in action this weekend, taking on last year's semi finalists Scarlets in Llaneli.
The Welsh side must win to keep their hopes of progressing alive, while a win for Ulster would bring them back into quarter final contention.
The Scarlets were comfortable winners when the sides met in the Pro 14 a fortnight ago, however both sides were without a number of frontline players.
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