Bolger returned to the Memorial Stadium for his second loan stint last summer on a season-long deal from Leicester and was joined in January by fellow Foxes defender Parkes and Stoke midfielder Lund.
And McGhee, who plans to release his retained list later this week, told the club's official website: "We are short of a left-sided player and at the moment we have no recognised centre-halves at the club.
"I am currently talking to Leicester about the possibility of bringing Tom Parkes and Cian Bolger back. I am also talking to another centre-half this week, and I have to make a decision on Aaron Downes.
"Up front I feel as if we need to freshen things up a bit. I have spoken to Stoke about having Matt Lund back as I think he is going to get better and better, and I would like another couple of midfield players."
Meanwhile, Rovers have announced a pre-season friendly against local rivals Bristol City for Louis Carey's testimonial. The clash, which is the first meeting between the two sides since 2007, will take place at Ashton Gate on August 4.