Albert Barnes Commentary 1 Samuel 31:7

"And when the men of Israel that were on the other side of the valley, and they that were beyond the Jordan, saw that the men of Israel fled, and that Saul and his sons were dead, they forsook the cities, and fled; and the Philistines came and dwelt in them." — 1 Samuel 31:7 (ASV)

The men on the other side of the valley - This must mean the area north of the plain of Jezreel, which would include the tribes of Naphtali, Zebulun, and probably Issachar. However, the text of 1 Chronicles 10:7 says, that were in the valley, limiting the statement to the inhabitants of the plain of Jezreel.

On the other side Jordan - This phrase most commonly means east of the Jordan, with the speaker presumed to be on the west side. But it is also used to refer to the west of the Jordan, as it is here, if the text is sound.

The Philistines ... dwelt in them - One of the principal cities, Beth-shan, immediately fell into their power (1 Samuel 31:10).