That's the whole point though - it is not a gain! What is happening here would be better described as "Eircom shareholders to crystallize losses on 50% of their holding".
The Eircom aspect is incidental. People who are Vodafone shareholders, whether thru' the Eircell sell-off, or through buying Vodafone share, are to receive unexpected money. The gain or loss aspect really depends on what a shareholder bought the shares for, or what price they were acquired.
Personnally speaking, I have these Vodafone shares as a result of the Eircom privatisastion 15 years ago. Then when Eircom was refloated ten or twelve years ago, I bought into Eircom again. Lovely dividend back then, 7%. And I sold them at a profit. Gain overall for me.