Briefly, the Southern League was regarded as almost on a par with the FL and the SFL. Spurs won the FA Cup in 1901 as a SL side and Southampton reached two FA Cup Finals in 1900 and 1902.
The FL consisted mainly of sides from the north and midlands. There were a few southern teams like Arsenal and later Chelsea and Spurs but the SL was a strong organisation in its own right. Strong enough to be included in official meetings with government during WW1 along with the FL and SFL over wartime football.
Certainly the SL were stronger opponents than the Irish League.
After the war the SL was adopted en masse as a new Third Division and a year later became the Third Division (South). Here's a
link to the first Third Division table. You can see from it the number of big cities it includes.
After assimilation the SL declined but still remained the top league outside the FL until the formation of the national Alliance Premier League (now the Conference) in 1979.