Things to consider, in order of priority:
You want a lawyer advice, in writing
Anything you will read here has 0 business value and 0 value in court. Written legal advice will at least show due diligence in following relevant work and anti-discrimination laws.
You must not discriminate on nationality
It is protected quality in practically all countries of European Union, and it is protected in united Kingdom
You are allowed to "discriminate" against not being able to perform the job
You need to be prepared to prove such duties are really relevant to the role you are hiring for. In this case, business trips to USA are legitimate, no company would pay for such trips if they weren't. Not for new hire, anyway.
Be careful to have "ability to travel to the USA in foreseeable future" as a requirement. No mention of Iranians whatsoever.
Be careful to whom travel ban applies.
I am not a lawyer, but as far as I understand having USA "Visa" in passport is totally irrelevant to the ban* - if it applies, even person with valid business visa will not enter. On the other hand, it does not apply to people who hold double USA / Iran citizenship. And it is complicated for people with other combinations of citizenships.
You do want to consult migration lawyer about that. Or, you can schedule interview in USA offices. It will prove that such person can enter USA at least once, and thus, that travel ban does not apply to her. More than once? USA never guarantees that anyway.
And with all that in mind,
You want a lawyer advice
Yes, I know I repeated myself here, but that's important, because this situation is complicated on so many levels one could make an academic degree on it. Even courts in USA couldn't agree for a long time in what parts Trump's ban is legal, and what measures to implement it are legal, and that's barely beginning of your problem.
From moral perspective, there is nothing wrong with refusing employment if candidate is plainly unable to do the job. Especially if reasons are beyond control and candidate cannot "improve" on them, and you couldn't accommodate or her inability in any reasonable way.
* OK, there was exception for Iran people with student visa, but not business visa. No exception for Syria people at all. Yemen visa holders could enter with specific ban for business and tourist visa holders... Maybe it have changed already. Have I told you to consult a lawyer?