Hi Josh
alerted I am, I live about 25 miles from Dundalk in Drogheda so have no native knowledge of the area. However I searched the Tithe Applottments for 1834 and found a Philip Mandavell (spelling of name varies but I think we would consider Mandeville the correct one) in Ballinfull, Roche parish, Dundalk. There are 28 hits so just put in Roche and Louth, also a Traynor, Heeny but no Magee.
see http://titheapplotmentbooks.nationalarchives.ie/search/tab/index.jsp As you are probably aware Irish records beyond these church records you found are almost non existent so we use other records just to trace the name and I see in the Hearth Money Rolls (a tax on fire places I think which still exist for some counties including Louth have a Mandeville in http://www.failteromhat.com/louthhearth/ffaghard.php I think now spelled Faughtart.
Also a very interesting site with a number of Mandevilles hits is http://www.jbhall.freeservers.com/ this site deals with Louth and gives lists and grave records, no definite way of telling if your family but indictitive of the name in the county.
The local library offer a service which if it costs will be small in case you wish to research a specific incident. In 1847 which was coming to the end of the famine the port of Drogheda was the 2nd biggest port for migration as there was a service to Liverpool and onward. I have answered a few queries from Wiscosin so it must be a popular place, was in Duluth once upon a time so it is famous for more than Bob Zimmerman or Dylan. See http://www.louthcoco.ie/en/Services/Library/Genealogy-Research-Sources/
There is a Flax Growers list for 1796 and I see a Mandiwel in Dromiskin which is near Roche. It may be a misspelling.
See http://www.failteromhat.com/flax/louth.php
In the 1641 to 1670 there was a land transfer to put it mildly and the owners and new owners can be seen here if you are interested.
http://downsurvey.tcd.ie/ The only Mandeville or similar is James in Tipperary and he appears to have held his land.
The online phone directory lists one Mandeville in Dundalk but landline phones are not as popular now with people so htere may be more, the local paper is the Dundalk Democrat or Dundlak Argus. See http://www.dundalkdemocrat.ie/ or http://www.independent.ie/regionals/argus/ in case you wish to write to them seeking assistance in tracing relatives.
Also for about $18 you can join the local history society and get online access to their material. See http://www.clahs.com/
I just looked in McLysaghts Surnames of Ireland and the de Mandeville came to Ireland in the 12th century, a Norman family and settled in Antrim (The Normans invaded Ireland in about 1170) some of htem moved ot Tipperary and Waterford and some became Mansfield, He gives a 16th century reference that they changed their name to Mandeville from de Maydewell at that time in Balydine in Tipperary, this is where the fellow is in 1641. If you like I can scan the entry and post it.
If you have any further queries or comments let me know,
Regards
Pat