In case you're interested, since I had a Gallagher grandfather myself, I can add some info about the Gallagher surname. It is believed to have originated separately in two places, in County Donegal and in the Mayo/Galway area. Since Monahan is in Ulster (though in the part located in the Republic of Ireland), your family may ultimately trace back to Donegal (looking for DNA matches may help to answer that question), perhaps ending up in Monahan during the Plantation of Ulster, when many people were forced out of the planted areas. In Irish, Gallagher is Ó Gallchobhair (modern Irish spelling Ó Gallchúir), and it means “descendant of the foreign help”, referring to (real or legendary) mercenary warrior ancestors who were brought in from Scotland or elsewhere in Ireland to fight in local wars (known as a gallóglach, or “gallowglass”). It may also refer to Norse ancestors, since the term “gall” was used for them as well. Although Gallagher and Gallaher are by far the most common anglicized forms of the name, especially in Mayo and Donegal, there are more than 20 other historical anglicized forms scattered in several places in Ireland.
The surname Smyth/Smith can have various origins. It may have been brought from Scotland or England, but in Ireland it is often derived from the Irish names Mac an Ghabhann or Mac Gabhann, which mean "son of the blacksmith". They were often anglicized as MacGowan, but also often "translated" into Smith, particularly in County Cavan, where both names are believed to have originated. Since Cavan and Monahan share a border, and Clones is not very far from that border, your Smith/MacGowan roots there may go way back.