I'm trying to find out the answer to this question also. So far I've had no luck in finding what the problem might be.
I have successfully swapped most teams from the Irish league to the English league (I'm creating a British superleague), but teams such as Distillery, Donegal Celtic, Ballymena United, Wakehurst and Chimney Corner are causing an "array access out of bounds" error.
I was thinking this was something to do with the number of allocated teams in each division, but the numbers seem to tally.
Also, I haven't had any problems with the Scottish and Welsh teams I've swapped.
Here is the full list of clubs I'm having a problem swapping:
Chimney Corner
Wakehurst
Dundella
Ballyclare
Knockbreda
Warrenpoint
Glebe
Distillery
Donegal Celtic
Ballymoney
Ballymena United
The database changes validate ok in the FM11 Editor. The game returns the error after I've selected all four leagues (Welsh, Irish, Scottish and English/UK). The error doesn't happen when I only select the 'UK' (English) league.
So my thinking so far is that the Irish league is somehow corrupted by swapping one of the English clubs into one of the Irish divisions.
Any thoughts?