Can an employer ignore a doctor’s note regarding a restriction due to an injury suffered outside of work?

First, the doctor has no legal authority over your employer, so the employer is not required to automatically do what the doctor says.

What the employer has to do is IF your condition rises to the level of being a "disability"--that is, something which significantly affects your ability to do basic life functions (e.g. walk or stand; lift weight; etc.)--and IF that disability affects your ability to do the job, and also IF you could do the job with a "reasonable accommodation" (see below), they'd have to make that accommodation.

A "reasoanble accommodation" is some change in how the job is done, or provision of some assistive device or technology, which is not too disruptive or expensive. An example is letting a cashier sit instead of stand; letting something with a colon or bladder problem have more bathroom breaks; letting an administrative assistant who has a hurt arm not have to lift heavy boxes of copier paper; etc.

So, it may be that your employer has to make some changes to how you do the job, or give you something to help you do the job, but that's only if it's reasonable for them to do it--and only if you can, with that help still do the job. If you simply can't do the job you were hired for anymore, they do not have to accommodate you.

Employers ignore requests by an employee's treating physician all the time in this country as to work restriction issues. The problem is that such ingoring can get the employer in trouble. If this is what is happening to you, then you should contact a labor attorney in your locality for help. One can be found on attorneypages.com.