I would like to bring my Nigerian girlfriend to ireland to live with me and I intend to marry her what do I have to do to get her legally into the country

She has to initiate from her country. You can't.

Have you ever met her or only talked to her online and over the phone?

DON'T SEND ANY MONEY!

Not only that she is subject to any waiting lists for immigration purposes... if she is in fact real and not some online girlfriend scam.

She might have to wait a signicant number of years.

Normally you will need to show a long term relatonship with her. Often that means both travel as tourist to the others country. So your first step will be for you to travel to her and meet her and get to know her family.