The Moriarty Tribunal has heard that former Communications Minister Michael Lowry wrote to the ESB in 1996 to tell it that it would have to share its masts with ESAT.
The letter followed a disagreement between the ESB and ESAT over whether it was government policy for companies like the ESB to make their facilities available to ESAT, which had just secured Ireland's second mobile phone licence.
The tribunal is currently investigating what prompted Mr Lowry to write the letter.