The Rohirrim
 

I'm reading LOTR for the third time this year and I've come upon a passage that I never noticed before. It's Book 4 Chapter 5 The Window On The West Page 322. To be exact. Faramir is telling Frodo and Sam about the history of Gondor. This particular passage caught my attention:

He later goes on to say that these people are the Rohirrim and also that they come from the Houses of Men specifically Hador. It's been a while since I've read the Silm but I don't remember reading that the House of Hador had assailed Gondor or any other house of Men. Does anyone have any information on this and is it in fact in the Silm if so where?

I do know that Eorl, first King of Rohan, made an oath. The oath was Rohan will come to Gondor's aid in time of need (the beacons), and in return, Rohan was given Calenharden. That is probably the "truce" Faramir was talking about. I believe the current steward at that time was Cirion.

If you want to read more on this check out the appendix in LOTR, the House of Eorl section.

As Boromir says, Eorl made an oath to Cirion to fight for Gondor in times of need. In return he and his people were given the land of Calenardhon to dwell in, which became Rohan.

As for the people who assailed the Gondorians - this is purely speculative on my part - they may have been dissidents related to the Rohirrim and perhaps related to the Dunlendings. I don't have the books on me now but the best answer I can give you is that the Rohirrim weren't descended from Hador himself; thus it's a bit ambiguous and therefore, no descendants of Hador himself assailed the Gondorians. However, those who followed Hador and lived among his folk were the forefathers of the Rohirrim, so while the followers of Hador may have attacked the Gondorians, his actual House (those who were directly descended from him) did not, as far as I know.

I've expressed this in a bit of a muddled way, but hopefully you get the gist of what I'm saying :).

I believe the predecessors of the Rohirrim originally were the peoples of Rhovannion (from whom the kinstrife eventually arose when Valacar married the daughter of the King of Rhovannion).
When eventually driven out of Rhovannion by the Wainriders they moved east
of the Anduin and then north past the Carrock, until Eorl the Young helped out
Gondor. Of course, during the Kinstrife they assisted Eldacar against Castimir.
(appendix)

Re: The Rohirrim
 

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!

The People of Hador, not just those from his House have their origins further back to Marach, as being the first to settle in Beleriand. Therefore they could be called the Marachians.

Northmen is quite a vague term since there was, in the Third Age, many sub-divisions that could trace their ancestries to the original groups of Edain who decided to settle in Rhovanion and about the Vales of Anduin during the journey West towards Beleriand.

We have the Wood Men, Beorn's people; the Lake Men and people from Dale etc.

You're right, Turin. If I recall correctly, it is even said in one place that the Éothéod, the ancestors of the Rohirrim, were closer related to the Men of Dale than to those of Gondor.