^ IANAL (i.e. this is not a legal opinion), but if the properly completed registration certificate that you have witnessed being posted to Shannon bears a 'signature' of the current registered owner in the appropriate (transfer of ownership) section, then, all other things being equal, a new registration certificate will be posted to you from Shannon.
In other words, anyone can scrawl a 'signature' on the certificate. You will only have to worry if there actually is something underhand going on (i.e. if it turns out that the actual owner had not consented to the sale), and I presume you will be able to get a gut feeling about this before committing to buying the vehicle.
On a separate point, you mention 'construction vehicle' in your post. If you mean that the vehicle is classed as commercial for motor tax purposes, you should make yourself aware of the new regime regarding the taxation/use of this class of vehicle, which makes it very difficult for an owner to use such a vehicle for any non-commercial purposes. Do a search of this forum for more information on this if it is relevant to you.