Well, this beauty of a card seems to have it all: shamrocks, harp, pipe, and castle ruins. Both the Celtic Harp and Shamrock are national symbols of Ireland, and the white clay pipe (women used them, too) is often found in images representing the country.
The publisher logo of letters S and B in a circle, are found on other postcards online attributed to the Samson Brothers. From a New York City directory, the Samson Brothers appear to be Joseph and Hy Samson, address given in the R. L. Polk & Co.’s 1915 New York City directory, as 67 Irving Place.
Divided back, embossed, unused postcard. Made in U.S.A. Publisher: Samson Brothers, Series 315 or S315. Date of card unknown, possibly circa 1915.
Source: R. L. Polk & Co.’s 1915 Trow New York Copartnership and Corporation Directory: Boroughs of Manhattan and Bronx. Vol. 63, pg. 908. (Google eBooks)