Definitely makes sense that a lifting train would need to have its own water supply once rail access to local water towers had been cut off.
Also, another reason why you might be seeing so many tanks turning up in pictures is that water hardness is a big issue in many parts of Ireland - so another use for water tanks would have been to bring in water from other areas with soft water. I think, for example, the GSWR/GSR used old tenders to supply Limerick Junction.
More info on the use of old tenders in this role is here: