The proportion of Roman Catholics in Northern Ireland is rising while the Protestant majority is shrinking because of a lower birthrate and higher Protestant emigration, according to a new study.

The ratio of Catholics to Protestants is a key issue in the British-ruled province of 1.5 million people. If one day Catholics are the majority, they could vote to unite the province with the Republic of Ireland.Britain has long said it would let Northern Ireland do so if that was the wish of the majority, though it clearly never expected it to happen. The borders of Northern Ireland were drawn by Britain in 1920 with the express intention of maintaining a pro-British Protestant majority there.

But the demographics are changing in favor of Catholics, according to a study by two professors at Dublin's University Col-lege. It was published Wednesday in the London School of Economics' journal, Population Studies.

An analysis of the 1991 census estimated that about 44 percent of the people in the province are Catholics, compared with only 33 percent three decades ago.

"There is a prospect of further significant increases in the proportion of the Northern Ireland population that is of Catholic back-ground," the study concluded.

Co-author Brendan Walsh said in a telephone interview that a higher proportion of Catholics "must be threatening to a (Protestant) population that was politically dominant and de-mo-graph-i-cal-ly always very strong."

Since the Catholic-based Irish Republican Army declared a cease-fire last fall, Britain has stressed long-standing pledges to British loyalists that there will be no constitutional change without "majority" consent.

Planned political negotiations on Northern Ireland's future envisage participation by Sinn Fein, the IRA's political ally, which seeks to unite the province with the overwhelmingly Catholic Republic of Ireland.