Portumna Castle was built before 1618 by Richard Burke, 4th Earl of Clanricarde—a 'semi-fortified house' rather than a true defensive castle, marking the moment when Irish lords stopped expecting to be besieged and started building for comfort. It served as the main seat of the de Burgo family for over 200 years until a fire gutted it in 1826. Today the ground floor is open to visitors from April to September, with the seventeenth-century walled kitchen garden restored to its formal layout. The grounds also contain the ruins of Portumna Abbey, a fifteenth-century Cistercian foundation. The castle holds two competing legends about an Irish Wolfhound. In one, the dog—called Fury, according to a marker stone—broke the fall of a child tumbling from an upper window. In the other, the older one preferred by the oldest local families, the wolfhound died on the balcony beside the Earl's daughter in the 1826 fire. Excavations have uncovered dog bones at the castle. They sit on display today, not entirely sure which legend they belong to.

Hubert de Burgh-Canning, 2nd Marquess of Clanricarde, was one of the most notorious landlords in nineteenth-century Ireland. From the comfort of his London apartments, he treated his Galway tenants with a coldness so total that the Irish National Land League launched its Plan of Campaign against him directly. 'If you think you can intimidate me by shooting my agent,' Clanricarde was quoted as saying, 'you are mistaken.' In November 1886, 4,000 tenants marched past his estate office at Portumna behind a brass band, accompanied by nationalist MPs who decided collectively how much rent should be paid. The tenants offered the agreed sum; the agent refused it; the money was handed to the MPs for the tenants' defence in court. The MP William O'Brien called the scene 'an artist in political melodrama.' The British response was the Perpetual Crimes Act of 1887—the last Irish crimes act ever placed on the British statute book.

The Portumna Union was formally declared on 22 February 1850, a subdivision of the Ballinasloe and Loughrea Poor Law Unions. By that point the Great Famine had already killed and exiled enormous numbers of the people the union was meant to serve. The workhouse, designed by the Poor Law Commissioners' architect George Wilkinson and built on an 8-acre site half a mile north-east of Portumna, was intended for 600 inmates. It cost approximately £7,875 to build and fit out. After the Famine eased, the buildings drifted in and out of use; by the late twentieth century the workhouse stood semi-derelict, with one front block serving as a local council depot. In the summer of 2011, the Irish Workhouse Centre opened in the surviving Portumna buildings—the only centre in Ireland devoted to telling the workhouse story. The institution that had been a final, brutal stop for the famine generation became, finally, a place to remember them.

Portumna Forest Park covers almost 600 hectares on the northern shore of Lough Derg—originally part of the Earl of Clanricarde's lands, purchased by the Irish government in 1948 and now owned by Coillte. Forest and lakeside walks thread through a mostly coniferous canopy interrupted by broadleaved stands, marsh, open ground, and offshore islands. Adjacent to the park, Portumna (or 'New') Harbour gives boaters access to the Shannon waterway system. Three Scout associations held a joint jamboree here in August 1985 to celebrate International Youth Year—10,000 participants, torrential rain, instant nickname 'Port-mud-na '85'. Today the town is a hub for boaters, anglers, and golfers, with Portumna Golf Club (founded 1913) winding around the parkland and a 100 km ultra-marathon weaving through the forest paths every year.

Portumna GAA has won the All-Ireland Senior Club Hurling Championship four times—in 2006, 2008, 2009, and 2014—and the Galway County Senior Hurling title six times. The hurler Joe Canning came up through this club before going on to a Galway senior career studded with All-Stars. Portumna Castle itself now serves as a heritage museum dedicated to the Flight of the Wild Geese—the Irish soldiers who left for continental Europe after the defeats at Aughrim and Limerick in 1691, taking service in the armies of France, Spain, and Austria for the next century. The Castle that watched the Wild Geese leave now houses their story. The hurlers who play in the GAA grounds across town carry the same surnames the Wild Geese took to Paris and Madrid. The oak whose landing place gave the town its name is long gone. The town that grew up around it is still here.