If Conor McGregor ever wants to make the jump to the squared circle, he'll have Ronda Rousey to thank ... so says Sheamus, who tells us Rowdy paved the way for MMA stars in WWE.

TMZ Sports spoke with the Irish superstar about Ronda's transition into pro wrestling ... and Sheamus says he wouldn't be surprised if more UFC fighters follow her lead.

"I think anything is possible now with Ronda in the WWE ... Ronda's killing it, so the door's open for anybody."

BTW -- Conor and Sheamus had a huge Twitter feud back in 2014 after the Notorious claimed he would "whoop his ass" in a fight.

But, it sounds like Sheamus isn't holding onto a grudge ... and tells us why he'd never be tag team partners with Conor.