The answers here have tended to refer to English laws/rules of heraldry whilst overlooking that the question relates to Irish heraldry, which like Welsh heraldry, had developed some rules of its own which predate the imposition of English rule.
Just as it is acceptable for a Scots clansman to display his Clan chief's Crest, or badge, (but not the Chief's Arms) so the Irish have the concept of Sept arms.
To quote from the Wikipedia entry on Irish Heraldry :
A distinctive feature of Irish heraldry is acceptance of the idea of sept arms, which belong to descendants, not necessarily of a determinate individual, but of an Irish sept, the chieftain of which, under Irish law, was not necessarily a son of the previous chieftain but could be any member of the sept whose grandfather had held the position of chieftain (tanistry). A member of the particular sept has the right to display the arms of that sept, a right that on the contrary does not belong to people of the same surname who belong to a different sept. For example, a person from the O'Kelly sept of Ui Máine may display the arms of that sept, but a Kelly of the Meath or Kilkenny septs cannot.
... the Irish Genealogical Office (previously known as the Office of Arms) "holds that any member of a sept may display the arms of that sept (as distinct from personally 'bearing' the arms, as on stationery, silver, or other such use, [for] only the grantee and his descendants may 'bear' the arms)".
If I understand this correctly it is acceptable for you to display the Sept arms of the two individuals linked in a marriage provided that these are the arms of the correct septs to which they belonged but you may not claim or use these as your own personal Arms.
To explore this further refer to the Chief Herald of Ireland.
Maec