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This thesis describes and analyses the results of surreys conducted during the summer months of 1963, 1964, and 1965 with an underwater North American gravimeter in the Irish Sea, together with certain detailed magnetic data obtained using a sea-borne proton magnetometer. In addition use has been made of the Aero-magnetic Maps prepared by the Geological Survey in order to co-ordinate and qualify the observed gravity field. For the purpose of this work the North Irish Sea has been defined as that area bounded in the south by a line approximately from Holyhead in North Wales to Dublin. A line running from the Moume Mountains to the Calf of Man, and from Ramsey on the east coast of the Isle of Man to Fleetwood formed the northern limit of operation. The surveys were directed towards extending the gravity coverage already accomplished in the north-east of the area. They provide further evidence relevant to the evaluation of the deep structure of the Irish Sea Basin. The low Bouguer Anomaly values known to exist in the north-east of the area as a result of an earlier marine gravity survey have been shown to extend over a wide area in the eastern part of the North Irish Sea. The original interpretation in terms of a deep sedimentary basin containing low density sediments was confirmed. A large negative anomaly was also discovered in the Dublin Bay area and this was again interpreted as a large sedimentary basin. A linear magnetic anomaly that appears on the Aero-magnetic Maps in the vicinity of Morecambe Bay was the subject of detailed traverses using the sea-borne proton magnetometer. It was concluded, that a sill or lavas dipping to the north with a direction of magnetisation towards the south could best account for the observed anomaly.