You did not mention any dates for your Thomas Jackman.
The Jackmans in Ferns appear to have been tied to the ones in Gorey. Of which there are a few people out there in the world of rootsweb who are researching this line.
Can you advise dates and if you have already checked memorials, igi, etc? There is a marriage of Philip Jackman to Mary Pender in 1844 on the CSI.