If you are going north the A865 is the way to go but the minor roads are well worth exploring.
There aren't many Gaelic words that have made made it into English but machair is one. It means land behind the beach. There are lots of beaches. Quiet stretches of sand deposited by countless Atlantic rollers. The fertility of the machair is due to a mixture of peat and windblown sand, peat is normally infertile due to poor drainage. In summer you will notice many wild flowers. The scenery in the Outer Hebrides is amongst the most beautiful in the world - the sea is very clear. On a sunny day when it covers the white sands of the west coast it has more colours of blue and green than you can imagine. The white colour of the sand is due to its high shell content (over 80%). The marine life includes dolphins, seals, sharks and whales. The Minch (between the Skye and the Outer Hebrides) is one of the best areas in Britain for whale watching. Seals are particularly common and will be seen sunning themselves all round the islands. Not surprisingly the Outer Hebrides are also famous for shellfish which thrive in the unpolluted waters.
The Outer Hebrides are also noteable for numerous lochs. Some of these are brackish and others are dark and acidic, water lillies are quite common and many have populations of trout and charr. Birds include dunlin, redshank, plover, lapwing and the islands are the last stronghold of the corncrake. Nearly every beach seems to have a population of sandpipers. The islands are formed on the oldest exposed rock in the world, Lewisian Gneiss. This is grey coloured with bands of white and dark minerals contorted by the pressure of the earth. These were formed over 3000 million years ago, similar rocks are found today in Canada to which this part of Scotland was once joined.
Harris Tweed is cloth that has been handwoven by the islanders of Lewis, Harris, Uist and Barra in their homes, using pure virgin wool that has been dyed and spun in the Outer Hebrides. This is the definition of Harris Tweed contained in the Harris Tweed Act of 1993 and it ensures that all cloth certified with the Harris Tweed Orb symbol complies with this definition and is genuine Harris Tweed, the world's only commercially produced handwoven tweed. For centuries the islanders of Lewis, Harris, Uist and Barra have woven the cloth: Clo Mhor in the original Gaelic- 'The big cloth'. As the Industrial Revolution reached Scotland, the mainland turned to mechanisation but the Outer Islands retained their traditional processes. Lewis and Harris had long been known for the excellence of the weaving done there, but up to the middle of the nineteenth century, the cloth was produced mainly for home use or for a local market. Nowadays of course it is exported all over the world. More of the cloth is in fact produced in Lewis but you can still see Harris tweed being woven in Harris on the west coast at Luskentyre - take the dead end road off to Luskentyre beach to get to them: Luskentyre Harris Tweed Co. tel: 01859 550261.
Causeways join South Uist, Benbecula and North Uist. The car ferry terminal for South Uist is at Lochboisdale, though you might miss this out if you are cycling north from Barra. There is a food shop and a hotel providing bar food at the junction at Daliburgh (Dalabrog). South Uist is the hilliest of the Uists and the road has a few easy hills as it works north. There are possible diversions west on lovely little back roads and you might like to include these loops in your route if you can manage a few extra miles.
It's worth noting that there are regular bus services on the islands and that the drivers are usually quite obliging and will take bikes. This could be useful if there is a strong headwind or it comes on rain! There is a small hostel/bunkhouse at Howmore, you need a sleeping bag, the nearest shop is a mile away. In May you would not find too much problem finding a B&B but booking in advance would be advisable during the school holidays. I've generally found that most B&B owners, even if full up, will be able to suggest that Mrs so and so, further up the road might have a space - they might even phone for you!