As far as we know in canon the answer is: we don't know.
In terms of non-canon, in the DS9 relaunch we learn that at least one Borg ship has encountered the Dominion. The Borg at that time (during the TNG period I think) couldn't assimilate a Founder. Not sure of any other Borg-Dominion meetings in books, comics or games.
If you look back to the second page you'll find another thread
where people discussed Borg vs Dominion.