Apparently it shouldn't have a "sh" sound or a slender s, rather it should have slightly different emphasis on the "ograi" part before the s, them vocals almost being silent?
The pronunciation seems to be an anomaly. According to the spelling the 's' is slender and ought to be pronounced with an 'sh' sound. On teanglann.ie all three dialects pronounce both 'díograis' and 'díograiseoir' with a slender 's' but all three pronounce 'díograiseach' with a broad 's'.
I first encountered this exercise almost a years ago, and I've read the comments every time I've encountered it since then, so I'm well familiar with it's anomalous pronunciation. I don't have problems with other broad/slender exceptions like seisiún or siosúr, and remembering how to spell them. I just have some sort of block with this word.
In the US, I believe attorney and lawyer are synonymous. I know there's a difference between solicitor and barrister in the UK, but in the US, all attorneys are lawyers and vice versa. Irrespective of what you call them, they can represent a client and argue their case before a trier of fact. Looks like they have separate roles in Ireland.
Hmmm. This seems to be a classifactorial clause to me. Couldn't you say this sentence as "Is díograiseach é an dlíodoir"?