In reply to:

In SAMPA, something like: /'deSl/ and /'twA:l/; and in English orthography, something like: deshl and twahl.

You've got "deiseal" exactly right, but O'Siadhail's Conamara Irish would surely render "tuathal" as something like "tu:@l" (the standard form would be "tu:h@l"). At any rate, the stress would fall on the "u", not the "a".

You mentioned "deas" meaning "south"

Note also the words "deas" meaning "nice", and "dea-" meaning "good". This relationship between "right-hand" and rightness, goodness, etc. is common to a whole bunch of IE languages and Finnish.