It think you are referring to:
The North Star, Polaris
Two guardians for a guide
Polaris was a guide for sailors in the past to locate the direction they were going
Radiant ladies on her side.
In the Ursa Minor Constellation, Polaris has two bright stars on its side, Kochab and Pherkad. These two are the brightest stars in the constellation apart from Polaris.
Oh back, the first preside;
Herb pac the second replied
Oh back is an anagram for Kochab and Herb pac where b is reversed to make d and c to k because they sound alike is an anagram for Kochab.
Both have notorious great pride.
This is because they are brighter than the other stars of the constellation and the nightsky so they would feel superior.
For id luny was cast aside.
Id luny is an anagram for Yildun and it is cast aside because it is located in the Handle of the constellation, far away from Kochab and Pherkad.