I had a good look through the Davidstown records and did not find any additional children for Thomas and Mary, but did locate a good match for their marriage in the same parish in 1840 - Thomas Howlin & Mary Kirwan married 2nd March 1840 (3rd entry left page). There are three witnesses - Peter Leacy, Martin Tracy[?] and John Neill.
The images for civil death records currently online on IrishGenealogy only cover from about 1892, but earlier images will be added. If you prefer not to wait you could order a research cert costing €4.
For reference here a link to the death for Michael in 1917 - reported age 65 (c1852), place of death Courtnacuddy townland, occupation farmer, married, cause of death Dilation of the Heart, 8 months, informant his wife Bridget. The couple appear at Courtnacuddy townland on the 1911 census with four of their seven children, and in the same townland in 1901.
Michael married Bridget Buttle on the 24th August 1886 at Boulavogue Chapel, his residence as Courtnacuddy and father is given as Michael .. possibly a mistake like the father's name of Andrew's baptism..?
There is a marriage of Eliza Howlin of Courtnacuddy in 1885 to Patrick Nolan at Davidstown chapel, her father's name is also given as Michael Howlin, small farmer
Griffith's Valuation which took place during 1853 for this area show a Michael Howlin (4th entry on the list) leasing a house offices (barns stables workshops etc) and land of just under 30 acres. No other Howlins listed with property in the townland that I could see. Maybe Thomas worked a farm with this Michael - or Michael was a middle name he went by instead on Thomas ?
p.s. there's a possible match for your Andrew Howlin in the Vermont, St. Albans Canadian Border Crossing records on FamilySearch - he is showen as arriving in 1909 [?] from Canada, with a planned destination of Detroit. The record gives his age as 59, place of birth as Davidstown, Co. Wexford, and father as James... father's name is difficult to read, but looks like James on that 1841 baptism.
