Chemical Bank of New York, the country's sixth-largest bank, announced plans yesterday to enter the lucrative Florida market through an investment in Florida National Banks and an eventual merger of the two institutions.

In a proposed $91 million transaction, Chemical thus joins a growing number of money-center banks that are poised to go nationwide once the barriers against interstate branching are lifted.

The deal calls for Chemical and Florida National, with headquarters in Jacksonville, to purchase through their holding companies 2.4 million shares of Florida National's stock from an unnamed major shareholder. The price per share will be 1 1/2 times book value of the stock at the time of merger, or no less than $38.

Chemical will buy 1.8 million shares of the stock of which 1.3 million will be turned into Series B non-voting common stock. Florida National will obtain the cash needed to repurchase about 700,000 shares of its common stock by issuing to Chemical shares of a redeemable preferred stock with warrants attached. The warrants give Chemical the right to acquire an equivalent amount of voting common stock.

The agreement is contingent upon approval by Florida National's shareholders. Bill Curtis, Florida National's vice president for marketing, said the chances of getting the major stockholder's approval are good.

A number of large U.S. banks already have established so-called Edge Act offices in Miami from which they conduct primarily foreign business. But this is believed to be the first merger of a money center bank with a Florida bank. "We're very excited about being number one," said Curtis.

Donald C. Platten, chairman of Chemical New York Corp., the bank's holding company, declared, "Florida National is an excellent banking institution with a significant retail presence in the Florida market, a strong balance sheet, solid loan portfolio, and quality personnel. Our partnership with Florida National is a key element of Chemical's strategic plan for national expansion during the next decade."

Florida National has assets of $2.4 billion, making it the fourth-largest bank holding company in the state. It has 26 subsidiary banks with a total of 93 offices. It had earnings per share of $2.21 in 1980. For the second quarter of 1981, earnings were 58 cents per share, up from 54 cents in the same quarter of 1980. There are now 7.5 million shares outstanding, but another million would be issued if a separate intrastate merger now in negotiation is consummated.

Chemical New York Corp. has assets of more than $46 billion. Chemical joins Chase Manhattan, Citibank and Marine Midland of New York, which recently have purchased equity interests in out-of-state banks with a commitment to merge when permitted by law.

The Douglas amendment to the McFadden Act prohibits bank holding companies from making acquisitions or opening branches across state lines. It is anticipated this prohibition will be lifted by Congress in the next couple of years.

In other banking news, the American Bankers Association yesterday wrote President Reagan, urging that the Depository Institutions Deregulation Committee, headed by Treasury Secretary Donald Regan, stand by its decision to lift interest rate ceilings on Individual Retirement Accounts and Keogh plans as scheduled in December. The thrift industry has asked for a stay on this decision. Recently Regan persuaded the DIDC to heed the thrifts' entreaty and rescinded a scheduled increase in passbook rates.