Lynch was supposed to battle Rousey at "Survivor Series" back in November -- but she suffered a serious injury to her face and was pulled from the match.

But, Lynch says she still has a sense of "unfinished business" with Rousey ... and wants to settle things once and for all on April 7 at MetLife Stadium.

Despite the rivalry, Lynch praised Rousey for all that she's done for women's pro wrestling, women's judo and women's MMA ... but warns that she "just hasn't faced a real test yet!"

Meanwhile, Lynch is training her ass off -- recently hitting up Conor McGregor's famous training center in Ireland, the Straight Blast Gym ... where she worked out with Conor's trainer, John Kavanagh.

"John Kavanagh put me through the ringer," Lynch said ... "It was great."

McGregor has raved about Lynch -- praising his fellow Irish superstar publicly and telling her to call "any time you need backup."

So, will the two connect and beat some ass as the greatest Irish tag team ever? Don't rule it out ...