McIlroy's caddie is a man.
If you're referring to this person:
That's Caroline Wojniacki, the women's tennis star. She and McIlroy were in a relationship prior to his current wife, and she caddied for him several times.
If you're referring to this woman:
.That's Erica Stoll, his current wife. She caddied for him in a par 3 competition associated with this year's Masters.
So there's definitely sets of photos of two different women in white Master's caddying outfits associated with Rory, but the Wojniacki stuff is from 10 years ago, and he was in a relationship with her.
The woman he was rumored to have a possible affair with is CBS Sports reporter Amanda Balionis:
Now before anyone makes a comment about his "type", let's not forget this little aberration:
Yup, that's Meghan Markle...
(My uncle is a huge golf fan, including any personal dramas the players are involved in, and I got myself an involuntary crash course in Mr. McIlroy's romantic history watching the final round with him today...)