Easterby, who turns 30 next month, will bring the number of Irish players in Sir Clive Woodward's panel back up to eleven, following Leinster lock Malcolm O'Kelly's departure due to an abdominal muscle injury earlier this week.
Dallaglio suffered a fractured right ankle when falling badly whilst trying to halt a 21st-minute Bay of Plenty attack at the Rotorua International stadium.
Easterby said: "Obviously I feel for Lawrence - he was looking so impressive - so to go out of the tour this way is a massive disappointment for him.
"But I am relishing the chance to be involved in a Lions tour and cannot wait to get out and join the lads in what will be a challenging tour."
Easterby, who is following in the footsteps of O'Kelly's replacement Simon Shaw, is due to link up with the squad in the next couple of days, with Munster flanker Alan Quinlan to take his place in the Ireland squad.
Quinlan, who lined out with Ireland 'A' in March, this afternoon flew out at short notice with the tour party from Shannon airport for their two-test (June 12, 19) trip to Japan.
The 6ft 3in Easterby, already on stand-by for the Lions, was preparing to check in at the Limerick airport when he received the call from Down Under.
The Scarlets skipper was originally cleared to participate in the Far East tour after sustaining a neck injury in the recent trial match - in which Quinlan made a try-scoring impression.
Revised IRELAND SQUAD (Japan Tour 2005):
Simon Best (Belfast Harlequins/Ulster) 11/02/78, 6 capsReggie Corrigan (Greystones/Leinster) 19/11/70, 44 capsLeo Cullen (Leicester Tigers) 09/01/78, 15 capsTrevor Hogan (Shannon/Munster) 23/09/79, -Marcus Horan (Shannon/Munster) 07/09/77, 28 capsBernard Jackman (Clontarf/Leinster) 05/05/76, -Denis Leamy (Cork Constitution/Munster) 27/11/81, 2 capsEric Miller (Terenure College/Leinster) 23/9/75, 46 capsMatt McCullough (Ballymena/Ulster) 09/09/81, -Johnny O'Connor (Wasps) 09/06/82, 6 capsAlan Quinlan (Shannon/Munster) 13/07/74, 21 capsFrankie Sheahan (Cork Constitution/Munster) 27/08/76, 22 caps
Tommy Bowe (Belfast Harlequins/Ulster) 22/02/84, 1 capKieran Campbell (Dungannon/Ulster) 06/07/79, -Girvan Dempsey (Terenure College/Leinster) 02/10/75, 58 capsGavin Duffy (NEC Harlequins) 18/09/81, 2 capsAnthony Horgan (Cork Constitution/Munster) 15/11/76, 4 capsDavid Humphreys (Capt) (Dungannon/Ulster) 10/09/71, 67 capsKieran Lewis (St Mary's College/Leinster) 19/08/80, -Conor McPhillips (St Mary's College/Connacht) 09/02/81, -Kevin Maggs (Belfast Harlequins/Ulster) 03/06/74, 69 capsDavid Quinlan (Northampton Saints) 04/01/78, -Jeremy Staunton (Wasps) 07/05/80, 1 capPeter Stringer (Shannon/Munster) 13/12/77, 56
Gary Brown (Blackrock College/Leinster) 01/11/80, -James Downey (Clontarf/Connacht) 23/03/81, -Ben Gissing (Clontarf/Leinster) 15/07/75, -Shane Jennings (Leicester Tigers) 08/07/81, -Chris Keane (Buccaneers/Connacht) 25/10/78, -Ronnie McCormack (Buccaneers/Leinster) 27/04/77, -Paul Shields (Ballymena/Ulster) 25/09/78, 2 caps