The pair join the Us on initial five-week deals, meaning that they will be with Paul Lambert's squad until early January.
Walker, 35, has been with the Hammers since the summer of 2004, having made his name during a lengthy spell with Walsall.
He has made just 20 appearances for the Irons and featured in the matchday squad once this season.
Tierney, 23, has been a regular for the Shrews this season, but has been allowed to move on and try to secure a longer-term stay with Colchester.
The former Oldham Athletic player has scored once in 91 appearances for Town, having arrived from Boundary Park in early 2007.