Spellings in Ireland were all over the place. I once saw a gravestones inscription for the same family and the surname was spelled 3 different ways.
Grffith Valuation took place in Cork in 1852:
Frederick Smythe, Parish of Holy Trinity, Cork City, Poor Law Union Cork, tenant of John W. Finch
#20 Patrick St. houses
No Frederick Smythe in Roscommon Griffith Valuation of 1852www.askaboutireland.ie
Frederick Smyth, Apothocary 26 St.Patrick St.
Smythe J. Morgan & Co. grocer and wine merchant 93 St. Patrick St.
There are other Richard Smythe as well as this sample:
Richard A. Smythe Carpenter & Cabinet Maker, Chapel Rd.www.census.nationalarchives.ie
The Tithe Applotments dated 1820-1839. You will be
ale to see if Frederick's parents show up.
Roscommon is one of the few counties where church records for certain RC parish's have survived in Ireland.www.familysearch.org
1911 UK Census Wealdstone, Middlesex
Frederick William Smythe, born Macroom, Co. Cork
Marriage 1847 Frederick Smythe Vol.9 Pg. 635 #4195887
I didn't look for Ellen, but the #'s should match.
I have an old set of CD from the Latter Day Saints that has the father's names, I will see if their marriage is there or not. If I don't get back to you don't hesitate to contact me, I have a brain like a sieve.
How people spent their lives is mostly in history books you can look to Eneclann, the genealogy division of Trinity
College who have books pertaining to each county.
I also have the 1871 UK Census that identifies only Irish
born. You say Frederick has passed by then and there are
139 Ellen Smythe's, so would need the county and you probably have that already.
Only 1 Smythe in the telephone book for Cork. 7 other counties and only 1 in Roscommon, might be worth a few