Problems displaying XY data correctly

Hi there.

I have a series of points marking areas just offshore into the sea that I'm trying to project as a new layer on top of my shapefile in ArcGIS, V10.1.
I have made a spreadsheet with the x and y values of these points. When I search for these points on google maps they display correctly, just off the southern coast of the Isle of Arran, Scotland, e.g.:

55.449269 -5.311943
55.428234 -5.205513
55.503167 -5.065094

I have a shapefile of the Isle of Arran which is using the British National Grid Coordinate system under Data Frame Properties.

What should I be using for the coordinate system of the input XY values? I have tried British National Grid, leaving them undefined, WGS 1984 and some others, yet whatever I do when trying to import the data points, they end up very very far away. Clicking the 'Full Extent' button after importing turns the Isle of Arran into a small dot as the other points are so far away.

Any help would be really gladly appreciated!!

Thanks.