I reckon that is the correct house. it (No. 49) is the 4th house from the end of Fitzwilliam Sq West which might account for it initially being called 4 West. I understand the four sides of the Square were built over a period of about 25 years (N, W, E, then S) so presumably the current numbering didn't come into use until after all 4 sides were completed. If you have the family name I can check Dublin Street Directories c. 1860 next week.