It also needs to be factored in that Ireland v Scotland fixtures such as Sligo v Motherwell tend to take place when the Irish clubs are mid season and the Scottish clubs are either still in pre season or right at the start of their league campaigns, which has definitely affected the outcomes of those fixtures.
Personally I would suggest that the LoI Premier is probably broadly on a par with the National League in England, other than the top end being closer to League 2 level. Down at the bottom end of the premier division you would find teams most years that would be more of the standard of the National League North and South.
