Town boss Mick McCarthy is understood to be targeting released Millwall skipper Alan Dunne. The versatile 32-year-old left the Den last month having been with the Lions since he was eight.
The Blues are believed to be in talks with the Dubliner, who can play at right-back, centre-half or in midfield.
Dunne progressed through the Millwall academy - having initially joined the club's youth set-up during McCarthy's time as manager - and into the first team, making his senior debut in March 2002.
The 5ft 10in tall defender went on to make 348 starts and 40 sub appearances for Millwall, scoring 23 goals, before his release at the end of the season.
Dunne, who was linked with Celtic earlier in the year, was named Millwall’s Player of the Year during the 2009/10 season in which they won promotion from League One via the play-offs.
In July 2011 he was given a testimonial match in which the Lions defeated Charlton 4-3 at the Den.
Another right-back is likely to be one of McCarthy's primary targets this summer with Elliott Hewitt having been released and with Jonny Parr increasingly having been used in midfield during 2014/15.