There are about 1,760 Irvings on Wikitree so far. We sure could use some more. First, go to the top of your Wikitree Page and search to see if your husband's grandma is already here. If not:
Sign up to FamilySearch.Org. It's a free site you can join that will help you search for your ancestor's vital records. It's pretty easy to sign up and familiarize yourself with. .
Always look for the location of that ancestor. It she's an Amy Irving from Boone County, Kentucky, you might google "Irving Family of Boone, Kentucky" and find that someone has written a book about your family history. They're not always completely accurate but they're usually accurate enough to be great leads.
Put an Irving surname tag on your Wikitree Home Page. This will keep you connected with others looking for the surname Irving. The Wikitree Community is extremely helpful with finding ancestors. They've not failed me yet.
And don't be afraid to ask questions. We've all been new here at one time or another and remember trying to learn the ropes ourselves.
The most important thing, though, is to remember to have fun. Finding details of our ancestors can be frustrating sometimes but, in the end, it's very rewarding.