Mirror.cs

using UnityEngine;
using System.Collections;

public class Mirror : MonoBehaviour
{

    public Vector3 ReflectPosition(Transform originalObject)
    {
        Vector3 localOriginal = transform.InverseTransformPoint(originalObject.position);
        Vector3 localReflected = Vector3.Reflect(localOriginal, Vector3.forward);
        var reflectedPosition = transform.TransformPoint(localReflected);
        return reflectedPosition;
    }

    public Quaternion ReflectRotation(Transform originalObject)
    {
        Quaternion rotation =  Quaternion.Inverse(transform.rotation) * originalObject.rotation;
        Vector3 axis;
        float angle;
        rotation.ToAngleAxis(out angle, out axis);

        axis = Vector3.Reflect(axis, Vector3.forward);
        return transform.rotation * Quaternion.AngleAxis(angle, -axis);
    }
}