These dates are early for Ireland.
Ardcarn/Toomna Parish records don't start till 1825-1900, marriages 1814 and deaths 1818. You should be able to get the children's births. A birth record should give you mother and father with mother's maiden name, parish/place and dates.
You have most of that already but you may uncover other children that didn't survive.
I can't remember what US Census gives date of immigration, how long they have been in US and naturalization. However, at their ages I'm not sure you could find them later than 1870.
The earliest record for land is the Tithe Applotments:www.genealogy.nationalarchives.ie
No William Johnston but 40 Johnston's and none in Parish
Ardcarne in 1834.
There are however, Frazer's in 1834 living in the Parish of Ardcarn. Most often marriages took place in the wife's
parish. First children often born there too as new mother's would go to their mothers home for the first and sometimes
In Griffith Valuation 1858 there are 9 Frazer's living in the Parish of Ardcarn, Poor Law Union of Boyle. In particular is Archibald Fraser with 4 listings as tenant of Viscount Lorton. Note an Alexander and a William but of different parish's.
You can see these at www.askaboutireland.ie
There is one Johnston in the telephone book for the area of Boyle, but no Frazer's. You could write a letter as addresses given are sufficient for mail delivery.