irregularities exceptions - Irregularity of あ-series in demonstratives

Why is the あ-series in demonstratives irregular like the following:

ここ　　そこ　　あ そこ　　　　　　どこ
　　　　　　　(expected あこ)
こう　　そう　　あ あ　　　　　　　どう
　　　　　　　(expected あう)

Is it related to the fact that こ, そ, ど all end with the vowel "o", and あ ends with the vowel "a"?

Answer

あそこ (expected あこ)

There was an ako. From the 1775 text 物類称呼 (Iwanami Bunko ISBN4-00-302691-8 p. 146):

あそこ　こゝといふを 西國にて・あんなけ こんなけと云 肥前にて・そこねい　こゝねいと云　尾州にて・あそこなて　こゝなてと云　京にて・あこと云

However, there are ample usages of asoko in much earlier works from the 13th century and on, so this is likely an abbreviation of asoko > ako.

Historically, in Old Japanese there was the ko/so/ka system. The ka form are the ancestor of modern a-forms, with the initial /k/ dropping out leaving only /a/. However, in the Old Japanese corpus, the ka forms are extremely rare and of those few that exist, they are all attributed to an eastern dialect. As such, it is thought that the distal (遠称) series had not been fully developed until yet early middle Japanese. It was a two-way system of speaker vs. non-speaker rather than priximal/mesial/distal. So that is one reason to not expect regularity in the [k]a-series. Further, the early signs of the [k]a- series are found in eastern dialects, so yo can expect variation when it merges with the western dialects.

Also, you seem to expect *あう based on こう, そう, and どう. However, I must take issue with こう and そう. The etymology of こう is kaku > kau > kɔː > koː, with a once medial -k-. For そう, it is sau > sɔː > soː. These two do not share a common etymology, so you cannot draw any conclusions from this alone for a ああ vs. あう. Besides, if it were あう, this would have regularly developed into au > ɔː > oː (おう).

As explained above, I think the question is based on a false premise so it cannot be answered satisfactorily.