The Rohirrim

[Nilpaurion Felagund]

I'll just add to what Fin2 already said regarding the House of Hador and the Rohirrim. The Rohirrim were not descendants of the people of the House of Hador, but their ancestors and the ancestors of the aforementioned Edain race were one and the same.

Now, regarding the attacks:

[Calmacil's] chief concern was with the Northmen. . . . The kings showed them favour, since they were the nearest in kin of lesser Men to the Dúnedain (being for the most part descendants of those people from whom the Edain of old had come) . . . but . . . the Northmen did not always remain true to Gondor, and some would join forces with the Easterlings, either out of greed or spoil, or in furtherance of feuds among princes.

LR Appendix A I iv - emphasis mine

There's the attack on Gondor Faramir might be referring to.

Hope this helps!