Dealing with these in reverse order
Rather than a Celtic League, I'm going to go the opposite way. I'm creating the Welsh league including all the Welsh teams. Makes more sense and makes an interesting Welsh league.
OK, although I would warn that this will create a more extreme SPL situation, with a Welsh Old Firm who would dominate.
Its ok, it needn't be a closed shop league. I'm building regional leagues. How would you bring it in though?
The Irish teams are a stretch, but doable. The only way I can think of Cardiff and Swansea is them getting kicked out of the English league and you would see more chance for relegation amongst the worst SPL sides.
I'm looking for a non-asb way. Consider it an AH challenge. Only way I can think so far is the Welsh sides getting kicked out of the English League in 1991 when the Welsh League was formed and as a result, Cardiff, Swansea, Wrexham and Newport all competing for honours. Even then, this would create problems with the formation of a Magners-style league. I've got the top division, but am stumped as to how it would be brought in.
Here's a back of a fag packet thought
1991 : Irate Eight don't break ranks, with the FAW's decision to ban those teams from playing home games in England immediately legally challenge this, and win. The Welsh Premier League becomes little more than a renaming of a merged Cymru Alliance and Welsh Football League National Division.
1992 : WFL starts to poor attendances and a strong lack of media attention. Meanwhile, in Scotland discontent is brewing as what supporters of other Scottish teams dub the "Selfish Six" of the Old Firm, Hearts, Hibernian, Aberdeen and Dundee United write to the SFL complaining about the distribution of sponsorship money, television revenues and the size of the league (IRL this happened about five years later).
1993 : First year of the Premier League in England. The Scottish Premier Divison notices a severe drop in interest. Cardiff City and Wrexham promoted to the English Second Divison.
1994 : The "Selfish Six" hand in a final set of demands, calling for a ten team Scottish SuperLeague, with total financial autonomy from the SFL, much like the Premier League, to start with the 1995-1996 season. The SFL reject this. Therefore, on Christmas Day 1994, the "Selfish Six" announce their resignations from the SFL, effective at the conclusion of the 1994-1995 season. However, the SFA continue to support these teams.
After being rejected from entry to the English football league pyramid system because of the potential reaction from FIFA with regards to merging the national teams, the Selfish Six have little choice but to try and form their own league. The six teams, now the newly formed Scottish Elite Football Clubs Association, discuss the possibility of forming a new league with the SFA. The SFA support the system. Meanwhile the FAW decide to exclude teams that do not participate in the Welsh football league system from the Welsh Cup, ergo removing the Welsh teams in the English system from a realistic chance at European Comptition. In a secret meeting in a hotel in Liverpool, the Scottish rebel clubs meet with Swansea City, Cardiff City and Wrexham. The Welsh clubs realize that the elite Scottish clubs would be more lucrative competition than the likes of Huddersfield Town. The Scottish clubs realize that they cannot run a viable league with only six teams.
An as-yet un-named Welsh-Scottish league is formed with nine clubs. A cash-trapped FAW, dealing with the zombie-like status of the League of Wales, which has lost money hand over fist, offer to sanction the Welsh-Scottish league, on several conditions. Firstly, the Welsh clubs agree not to participate in any competition, save for UEFA ones, that involve English clubs (namely, withdraw from the FA Cup, League cup and English league system). Secondly, they agree that a fixed proportion of league revenues, half that that the SFA recieves, will go to developing Welsh football. Thirdly, that there will be a mechanism of promotion and relegation from the (as yet untitled) Welsh-Scottish league. In return, with UEFA permission, the league slots in European competition will be awarded in the joint Welsh-Scottish League, treating each country separately in terms of positions.
1995 : First season of the Scottish-Welsh league starts, with the three Welsh teams and six Scottish ones.
Not sure about how to get the Irish teams in apart from just a commercial decision. I'll post ideas.
EDIT : In my older version, I'll try to rig up a system where Cardiff City and Swansea City are plonked in to the SPL at the expense of Dunfermline and Dundee (Not United), who in turn will go down to SD1, with such a domino effect downwards. I can't guarantee I'll actually be able to run anything much at all with it though.
EDIT2 : Also, Wrexham instead of Livingston.