When I was pregnant with my last one, I got one of those small calendars where you can insert pictures for each month. I wrote the birthdays of all my kids, me, dh, etc and put a picture that corresponded to whoevers birthday or holiday was in that month. For the month that I was due I put my most recent ultrasound picture and on the due date wrote "Baby #5: Boy or Girl??" and sent it to my mom!!!
You could find a shamrock or St. Patty's day frame and mail the framed ultrasound pic. You could also buy some St. Patty's day baby things (like a bib, socks, etc) - I always see stuff like this in Target - and mail them to your family along with a "I love grandma, etc" bib or onesie.
Good luck
I like the idea about the slide show on the computer... you can personalize it anyway that you want! I kinda wish that I had waited to tell dh and my parents but since we had been trying so long when I found out I immediately told my mom but waited a few weeks to tell dh's. It really wasn't a shocker for anyone lol. I know you will find the most perfect way to tell both sets of soon to be grand parents!