Hypnos
L'Empereur
Mmm, yeah.... unfortunately, I can't imagine that the Noble Houses of the Riverlands would have allowed any Lannisters to remain on their lands once they usurped the Freys/Lannisters at the end of the war. If you seek clarification however, we could just ask Hypnos.
Darrys position comes from the fact that no true born living Darry was left alive after the conflict of the war of five kings. Without an heir the castle was given to Lancel Lannister through the claim of his wife Amerei Frey (Whose mother was a Darry) Lancel would later join the faith and relinquish this title. This would leave only a few options for heirs: Amerei's next husband (presumably Lancel's younger brother), Amerei's sister and her family the Boltons, her other sisters: more Freys, or the children of Cleos Frey and his Darry wife.
This means that Darry will either fall to a Lannister, Frey or Bolton. Out of those options Lannister just seems like the most reputable name.