Statutory registration of births only started in Ireland in 1864. To find any siblings you’d need to search church records, if they have survived. Do you have the 1846 marriage certificate? If so, that will tell you the church where the couple married. It was traditional to marry in the bride’s church and so that is where I would look for her baptism and siblings. The marriage certificate should have both the bride & grooms townlands (addresses) and that will also help you find his church (assuming it wasn’t the same as the bride’s).
If you don’t have the marriage cert, it was registered in Ballymoney 1846 Volume 2, page 227. You can get a photocopy of it for 4 euros from GRO Roscommon. www.groireland.ie/ Quote the place, year, Volume & page number on the application form.