no CB Circassia is better than no CB Thomond for any other nation.
Why?
No-CB any Irish minor => vassalize => inherit war => conquer neighbours, make CBs => conquer any remains of Irish minors all this within 5-7 years from the game start.
This gives you an immediate powerbase in Ireland (except Pale), then you simply eat Scotland, then England, then BAM! you are a Great Power on the British Islands.
The only bottlenecks are:
1) unfavourable alliances at the start of the game = easily overcome by restarting before January 1448.
2) possible enforce peace from England = overcome by quick sieges.
Then it's only a matter of
Luck of the Irishyour skill, as you can fight England on your own (even with the Irish island only, but it needs a bit development).
The strategy works even for Muslims like Granada, although you need to change to Catholic for easier alliances.
On the other hand, Circassia is poor land with increased coring cost, surrounded by the same poor lands mixing multicultural Sunni/Orthodox/Shia/Coptic religion.
Besides, you are in immediate danger of early QQ, GH/Crimea, which fight a way better in plains than the English at the game start. And, of course, the neigbouring Ottomans (no need to comment)
How can all this be better?
Moreover, in terms of AE none cares for the Irish, then Burgundy and Brittany and Provence (if has territories) begin caring for Scotland, and some German minors care for England.
Contrary to that, conquer Muslims around Circasia => Ottomans, QQ, Timurids and Mameluks and tons of minors will care for your actions (more for Suni, a bit less for Shia). Conquer Orthodox, Ottomans and QQ will all care as well, because they are your immediate neighbours.
I don't say no-CB Circassia is impossible, I say it gives less time/benefit compared to Ireland for most coastal minor nations.