I'm going to ask the girl I'm dating to change her bra
Are you really going to do that? Why would you?
Because it irritates me. She looks better without it
It's very rude, and disrespectful, to ask any woman to change her underwear just to please you.  You should allow your girlfriend to dress in a way that she finds comfortable and pleasing.
I'm going to ask her to change it. She doesn't need to wear it. Maybe she will look cuter.
I understand you want to be very intimate with your girlfriend, but don't you think she would feel uncomfortable being demanded to change to your needs? How about just letting her wear what she likes, and you can gift her something the both of you might like.