With the 1861 census, James appears on sheet 208/image 35; John, and mother Elizabeth on sheet 209/ image 37; Daniel on sheet 210/image 39; and Thomas on sheet 213/image 45. I'd found these by accident, hunting through the pdf's a few years ago. I'd written it down on a paper and I hope it still is current info.
County Meath? I have no direct confirmation of their original home in Ireland. I have seen the County Meath (Merth, one place) reference but have never been able to confirm it definitively nor independently.
I have gone through the book you mention (Robert Sellar?) I'd found it on google-books last year. My memory of it isn't very clear but I recall a single reference to a Donahue (O' ghue?...)who probably would have been one of those original brothers.
Oh, and you may want to search the "1842 Census for Godmanchester Township" for the name 'John Dunachoe'. Almost certainly it refers to your John 1812-1866, married to Margaret Walsh Sept 9, 1839. By 1842 their first chidren: James (June 11, 1840) and possibly Margret (spelling in the old bible, April 25,1842) had been born. Your Thomas shows in that bible as having been born Feb 18, 1855.