Re: looking for Thomas O'donnell and Catherine Curran
You know when they were married.You could obtain a copy of the marriage record and note the place of marriage.The church in which they married may have further records.Be sure to inquire about ecclesiastical records for the couple.Occasionally you'll get lucky and they'll have noted where other sacraments were given, like baptism.
You know when they both died.There are death records.Copies of those will tell you where they were buried, and perhaps reveal other information like their places of birth, parents' names, etc.
You should be able to obtain copies of the marriage and death records from the civil records office in the area they were married and from where they died.
Records in Ireland and spotty at best for the 1840s. There was no civil registration of births, marriages, or deaths at that time.Without a church record to search, you're not likely to find much.Church records tend to be spotty for that time period as well, as that was right during the Great Famine and people were struggling to stay alive.
But you ought to be able to find some information about them from American records which could lead you back to the Old Country.
There are two great sources of info - one is the Latter Day Saints Family History Libraries.They have microfilms of many, many records from all over the world.They're free to look at, and if your particular library doesn't have a film you need they'll order it for a nominal fee.
The other is your local public library.Many libraries have a subscription to Ancestry.com and sometimes to other genealogy records sites.You can search them for free at the library.Both places almost always have someone who can assist you and get you started.