Okay, before I get chastised, let me just come right out and admit that I plan on driving from Dublin to Dingle in a single day. Yes, I know that has been ill-advised by many, and I know it'll take 5-7 hours, but it's really the only way we'll be able to do/see everything we want during our very short trip. So my question is this: what are some great places to have a brief stop along the way? Places to eat? Notable castles en ruoute? How about a castle where we can eat and kill 2 birds with 1 stone? We want to break up this long drive with worthy "pit stops". Thanks.

King John's Castle in Limerick has a small cafe. You can tour the castle and then eat.

FOOD FOR THOUGHT:
The most direct route is the N7, via Kildare, Port Laoise (Port Leash), Nenagh and Limmerick, but that would take you over the Connor Pass at the end of the day, possibly after dark, depending upon the time of year.
There is a castle at Nenagh, although I am unfamiliar with it.
A viable option, IMHOP, would be to turn off the N7 at Port Laoise, onto the N8 and go to Cashel and visit the Rock and grab a bite. A few miles further along is Cahir, which has a very nice castle (Parts of the movie "Excaliber" were filmed there).
At Mitchelstown, take the N73 to Mallow (Malla) and pick up the N72 to Killarney. You could turn off at Boherbue on the R577 and follow it through Ballydesmond and take the turning for Farranfore, where you could pick up the R561 into Dingle Town, or stay on the N72 into Killarney and then go north on the N22 and pick up the R561.
My way is probably longer, but it is, I think, more visually interesting. Since you have conceded the day to driving, why not make it an interesting one?

It's a drive but do-able - about two-thirds of the way toward Dingle, Adare is a beautiful little village with shops and restaurants. In October we drove from Dingle to Malahide (north of Dublin) we didn't stop in Adare as we had stopped there when we were heading to Dingle from the north (not Dublin). There wasn't anything of great interest IMO, other than what has been mentioned by others. By staying on N86 you can get to Dingle from Limerick and Tralee without going over the Connor Pass if it is getting dark.

Back again.
Did some checking on the Bord Failte web site (for some reason, the AAROADWATCH.IE site wouldn't display), where I plugged in the various routes from Dublin to Dingle into the route planning function. Interesting results.

Via Limmerick, 214.11 miles at 5 hours and 21 minutes.

Via Cashel, Mallow and Killarney, 228.64 at 5 hours and 59 minutes.

Via Ballydesmond, Cashel, etc., 224.59 miles and 5 hours and 55 minutes.

Distances and (ESPECIALLY) times are slightly suspect, but should be fairly close. Does not (of course) allow for meal and/or touring stops, but as an approximation of DRIVING time, it is probably fairly close.

Would not be a problem during summer months, but Nov-early May, you might drive some of it in the dark.

Hi logline:

It will be a long drive and hopefully it will not be your first day in the country after an overseas trip. Driving that distance with little sleep is a very bad idea. If it is your first day, you should plan many little stops.

I like Italian's idea for going through Cashel. It would let you avoid Limerick which holds multiple opportunities to get lost. I wouldn't spend the time going to Limerick and finding King John's Castle. Nice enough place, but the Rock of Cashel and Cahir are much more interesting and don't require getting lost. I also think that I would go through Mallow just because the roads are secondary national roads as opposed to regional roads. Again, less chance of getting lost.

As far as eating, I would stop at whatever town you happen to be in when you get hungry and grab some pub grub. It can be difficult to find food between 2:00 & 6:00 PM, so keep that in mind as you travel.

Bill