SOED seems to be telling me that curragh meaning a marsh and curragh meaning a boat are not related, but, by Jingo, that can’t be right, can it?

Apparently it is. OED makes no connection either.

currach, -agh

small boat, etc

[Ir. curach boat, little ship; also corrach boat, coracle; cf. Welsh corwg, also corwgl, cwrwgl CORACLE; these point to an OCelt. *kuruk-os, *kurok-os boat. (The spelling carrogh in Camden and his citers is prob. only a misprint.)]

curragh

marshy waste ground

[Ir. corrach marsh, Manx curragh moor, bog, fen.]

Homonyms are common; why the astonishment?

Well, in this case there is something of a semantic connection between the homonyms.