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
Just did this my side (Arc10.4.1), loaded Nat geographic base map, zoomed to Ireland, then set the data frame coord sys to Irish national grid, then set either of these - TM65_To_WGS_1984, TM65_To_WGS_1984_2 as the transformation (TM65 is a Modified Airy based system). So, was just going in the opposite direction to your case.
No probs here.
So what version you on?
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?
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).