Connect expects to record a 12% increase in the numbers it will help this year but counsellors are only able to answer 80% of the 8,300 calls it gets.
The out-of-hours telephone-based counselling service supports adults who experienced abuse, trauma or neglect in childhood from 6pm to 10pm every Wednesday to Sunday.
Connect manager Theresa Merrigan said while the rates of calls being picked up have improved, too many remain unanswered.
"The 2013 figures show that the service has challenges with capacity," she said.
"Some 20% of overall calls are received when the service is closed and 20% of calls received during opening hours are not answered due to staff capacity."
Last year trained counsellors answered 75% of 7,376 calls it received. More than 3,500 people also tried to make contact when the helpline was closed.
Four out of ten survivors suffered abuse from a family member - including sexual abuse by a brother, father or uncle or grandfather - while a quarter of cases happened within the community.
Institutional abuse accounted for 16% of calls since last September but that figure spiked to 22% when the report on the Magdalene laundries was published earlier this year, triggering people to look for support.
Women accounted for 64% of callers, 35.5% were male and 0.5% transgender.
Elsewhere, calls from people in the UK, where many victims have moved, doubled to 3%.
Ms Merrigan said an increasing number of callers to Connect talk about isolation.
"Caller presentations are becoming more complex as people fall out of the net around other services available," she said.
"Many callers present with psychiatric histories and are not availing of other professional or community support.
"Isolation is one of the key issues that callers talk about.
"One of the impacts of abuse in childhood is that many find it difficult to maintain relationships as adults.
"Very often there is a family breakdown."
Connect was established in 2006 following demands from groups representing survivors of institutional abuse that an independent and professional out of hours telephone based counselling and support service be established.
Counsellors are available Wednesday to Sunday from 6-10pm at 1800 477 477 and from the UK at 0800 477 477 77.