Boss (29) is at an advanced stage of negotiations with the European champions after five years with Ulster, where he has been extremely effective without it being reflected at national level (12 caps), having made his debut for Ireland in 2006. He would be a huge loss for Ulster both as a team leader and a versatile player, but if they can plug the gap with Blair, who toured South Africa with the Lions last summer, then they would be swapping one quality scrum-half with another.
It is understood that Ulster, who have secured fresh cash to invest in recruits, have already talked terms with Blair, who was co-captain of Scotland in their autumn series.
He is not the only player on their shopping list however, and South African Ruan Pienaar is also understood to be in the mix. If they land Blair they would gain in technical scrum-half play what they would lose in Boss's versatility.
The arrival of Boss in Leinster will give them a powerful scrum-half combination along with Eoin Reddan, who arrived from Wasps last summer with a view to improving his international chances.