My input for ConTeXt is:

   \enableregime[mac]
\usetypescriptfile [adobekb]
\setupencoding [default=ec]
\usetypescript [adobekb] [\defaultencoding]
\usetypescript[palatino][\defaultencoding]

\starttext

   Howth  {\em H\k{o}f\dh i} / {\em H\k{o}f\dh a},
...

Output from the console says that \dh is an undefined control sequence.
This and similar commands are mentioned in Kopka and Daly: A guide to 
LaTeX2e p. 423.
In LaTeX using \dh \DH \th \TH I get the result below. But the \ng \NG 
produce the black rule. I would be happy if just the \dh worked.