I agree for proper English it should be "a sunburn" or "sunburned." This is translated into bad English as is.
"a sunburn" is accepted. I think the unnatural feel came from the lack of a definite article in the Irish.
That would be more natural in English. "Griandó" is a noun so I suppose DL is requiring a literal translation.
Is there some grammatical practice in the caighdeán that causes it to be pronounced as "gríandó" (i.e. with a fado on the vowel "i")? Or is this just a regional canúint of the speaker?
The sequence of vowels ia is pronounced with a long i and a short a.
Examples: bia, iad, iasc, grian, Cian