It was taken over by the Kylsant Royal Mail Company in 1917 and renamed Coast Lines which by the end of 1917 held all the shares in the B&I.
Among the operations of this group were, The 1930s was a difficult period for the B&I, and Coast Lines offered the Irish Government a share in the company but they declined.
During the war, the company sustained casualties with the separate losses of two vessels in Liverpool in 1940: the Innisfallen, and Munster sunk by a mine.
The new management commenced a major programme of modernisation, launching the car ferries MV Munster (1968), Innisfallen and Leinster (1969).
On 25 April 1980 a jetfoil service from Dublin to Liverpool started but was withdrawn as it was not a commercial success.