Cannot transform TM_65 to GCS_WGS_1984 - Help!

Question

I have imported a shapefile of Northern Irish DEA boundaries, default projected at TM_65 Irish grid coordinates. I want to re-project them to GCS_WGS_1984. However, my Project tool tells me that all available projection changes (eg. TM65_To_WGS_1984, TM65_To_WGS_1984_2, etc.) are invalid transformations. How do I fix this error?P

MelitaKennedy · Answer

Try redefining the data to Esri's version of the coordinate system. If the GCS or datum names don't match, it won't find the available transformations.

NeilAyres · Answer

No, the data frame will assume the coord sys of the first layer added to it, if this is a new mxd.

Most web layers are in that Web Ax sphere thingy. But WGS84 based (the underlying datum / spheroid).

AdamBrodie · Answer

I'm using Arc10.5, and the Open Street Map basemap, which doesn't have a data frame coordinate system attached...but the boundaries of which best fit WGS_1984. Do I perhaps need to define the coordinate system?