He succeeds Joan Burton, who resigned following the party’s disastrous general election result.
Mr Howlin denied that he is a “reluctant” leader but said he is committed to rebuilding the party.
The press conference at which his election was declared was overshadowed by the absence of Tipperary TD Alan Kelly, who was unsuccessful in his bid to become party leader.
It is an extraordinary privilege and honour to be elected Leader of the Labour Party-Brendan Howlin pic.twitter.com/cZ2d1UFJ3e— The Labour Party (@labour) May 20, 2016
Mr Howlin’s name was the only name to go forward after Mr Kelly failed to secure a seconder among the six other Labour TDs.
For several days, Mr Kelly, nicknamed ‘AK-47’, sought the support of at least one of his colleagues to second his nomination but they resisted his advances, plumping for Mr Howlin instead.
Mr Kelly is popular among the party grassroots but his parliamentary party colleagues were openly hostile to his bid to become leader.
Newly-elected Labour Party leader Brendan Howlin pays tribute to Joan Burtonhttps://t.co/nL7kA8piJo— RTÉ News (@rtenews) May 20, 2016
Mr Kelly said the “wishes and entitlement” of Labour party members were denied by the lack of a contest, but said he wished Mr Howlin well.
“I wish Brendan Howlin well as leader of the Labour Party and congratulate him on what is a proud day for him, his family, friends, and supporters,” he said.
“The rest of the parliamentary party took a different view to what I believe is the members’ wish and entitlement. I want to thank the huge volume of members who have confirmed that view to me,” Mr Kelly said.
Despite disagreements, we all need to support the revival of our great party. Warm congrats to Brendan and all his team in Wexford @labour— Alan Kelly TD (@alankellylabour) May 20, 2016
Former party leaders Eamon Gilmore and Pat Rabbitte congratulated Mr Howlin on his election.
While not indicating his intentions as to whether he would remain in the party, sources close to Mr Kelly said he was unlikely to walk away.
For his part, Mr Howlin paid tribute to outgoing leader Joan Burton, saying “history will remember her contribution kindly”. He also said Mr Kelly’s absence at the press event was a “huge disappointment”.
Mr Howlin said: “We have just come from a parliamentary party meeting and Alan wasn’t in attendance. I texted him very early this morning. I know Alan to be a friend and a colleague, a man of great passion and ability, and I hope he will be a very integral part of advancing the Labour party.
“Would I like every Labour party member, activist, to be part of the revival? Yes, and I hope Alan will be.”
He added that he had not received a reply from Mr Kelly but when the conference ended Mr Howlin said he had received a text from the Tipperary TD which read: “Best of luck Brendan, it’s a special day for you and your family.”
Mr Howlin paid tribute to former Labour leader Brendan Corish and his father John, who he said were instrumental in developing his links to the Labour Party.
New Labour Party leader Brendan Howlin said the party did a lot of 'soul searching' after the general election https://t.co/jhYWAwyZ7o— RTÉ News (@rtenews) May 20, 2016
Former party leader Ms Burton said a huge number of party members had indicated that they did not want a leadership contest.
Mr Howlin, 60 and unmarried, has served as a TD for the Wexford constituency since 1987 and was a member of the Seanad before that.